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+## ehrQL error messages
+
+If an error is found in your dataset definition.
+ehrQL will stop running and give you an error message.
+ehrQL error messages are shown as a [Python error report, known as a "traceback"](https://realpython.com/python-traceback/).
+
+:notepad_spiral: The error messages are from Python because ehrQL runs in Python.
+
+These error messages can be confusing to read,
+but they also give you lots of information to use to debug
+and fix your dataset definition.
+
+### Example error message
+
+Let's look at an example of an error report:
+
+```pytb
+Traceback (most recent call last):
+  File "/workspace/analysis/dataset_definition.py", line 7, in <module>
+    dataset._age = age
+    ^^^^^^^^^^^^^
+AttributeError: Variable names must start with a letter, and contain only alphanumeric characters and underscores (you defined a variable '_age')
+```
+
+* The *traceback* tells you what code actually caused the error.
+  The traceback shows both the filename
+  and where the error occurred in the file.
+* There is an error message at the end.
+  The error message shows what kind of error occurred —
+  here, this is an `AttributeError` —
+  followed by details of what the problem is.
+
+## How to use this page
+
+### Structure of this page
+
+For each error, there is:
+
+1. a simple code example that causes the error
+1. the error details
+1. the simple code example modified to fix the error
+
+### Finding an error on this page
+
+If you are working with ehrQL,
+and encounter an error,
+this page may help you.
+
+Because of the included code examples and errors,
+this is a long page.
+
+Here are some tips on narrowing down the search
+
+#### Using the table of contents
+
+Skimming the table of contents navigation bar on the right-hand side of this page,
+to see if any of the general descriptions of errors apply
+to what you are trying to do.
+
+#### Using your browser's "Find text in page" feature
+
+Using the "Find text in page" feature of your browser,
+searching for parts of the error report.
+Let's look at the example given above again:
+
+```pytb
+Traceback (most recent call last):
+  File "/workspace/analysis/dataset_definition.py", line 7, in <module>
+    dataset._age = age
+    ^^^^^^^^^^^^^
+AttributeError: Variable names must start with a letter, and contain only alphanumeric characters and underscores (you defined a variable '_age')
+```
+
+The first part of this traceback depends on the specific code that has been written here.
+It shows:
+
+* the name of the file —
+  `dataset_definition.py` stored in the `analysis` directory
+* the line number in the file causing the error —
+  line 7
+* the line of code causing the error
+
+All of these will vary depending on the code being run.
+These are useful to point you to where your error is.
+
+However, they are possibly less useful to search for in the list provided here,
+because this part of the error report will vary.
+What *will* stay more constant is the final error message.
+Searching in this page for parts of that line,
+for example `AttributeError` or `Variable names must start with a letter`
+may show you the relevant error.
+
+:warning: This page covers many of the common ehrQL errors you may see,
+but is not an exhaustive list.
+
+:warning: Notice that even the error message may contain references to the precise code.
+In this example: `you defined a variable '_age'`.
+
+:question: Can you find the part of this page that does explain this error?
+
+
+## Python syntax errors
+
+These can occur because Python has its own syntactic rules
+that ehrQL code must also adhere to.
+
+### Code indentation error
+
+Python has particular rules about indentation.
+If a dataset definition contains indentation errors,
+the error message will tell you about them.
+For example, there is an indentation error in the following dataset definition.
+
+#### Failing dataset definition :x:
+
+```python
+from ehrql import create_dataset
+from ehrql.tables.core import patients
+
+dataset = create_dataset()
+dataset.age = patients.age_on("2023-01-01")
+ dataset.define_population(dataset.age > 16) # This line has incorrect indentation.
+```
+
+Run the dataset definition with:
+```
+opensafely exec ehrql:v1 generate-dataset analysis/dataset_definition.py
+```
+
+#### Error
+
+```pytb
+Error loading file 'analysis/dataset_definition.py':
+
+  File "/workspace/analysis/dataset_definition.py", line 6
+    dataset.define_population(dataset.age > 16)
+IndentationError: unexpected indent
+```
+
+The error message tells us that there is an indentation error, and also the line that
+the error occurred on.
+
+#### Fixed dataset definition :heavy_check_mark:
+
+```python
+from ehrql import create_dataset
+from ehrql.tables.core import patients
+
+dataset = create_dataset()
+dataset.age = patients.age_on("2023-01-01")
+dataset.define_population(dataset.age > 16) # This line now has correct indentation.
+```
+
+### Forbidden feature names
+
+Python has constraints on allowed variable names, which also apply to the names of dataset features.
+For example, a name — `age!` — with a non-alphanumeric character is invalid:
+
+#### Failing dataset definition :x:
+
+```python
+from ehrql import create_dataset
+from ehrql.tables.core import patients
+
+dataset = create_dataset()
+dataset.age! = patients.age_on("2023-01-01") # age! is an invalid feature name.
+```
+
+Run the dataset definition with:
+```
+opensafely exec ehrql:v1 generate-dataset analysis/dataset_definition.py
+```
+
+#### Error
+
+```pytb
+Error loading file 'analysis/dataset_definition.py':
+
+  File "/workspace/analysis/dataset_definition.py", line 5
+    dataset.age! = patients.age_on("2023-01-01") # age! is an invalid feature name.
+               ^
+SyntaxError: invalid syntax
+```
+
+#### Fixed dataset definition :heavy_check_mark:
+
+```python
+from ehrql import create_dataset
+from ehrql.tables.core import patients
+
+dataset = create_dataset()
+dataset.age = patients.age_on("2023-01-01") # We have changed the invalid feature name, "age!", to a valid one, "age".
+```
+
+## Common ehrQL errors
+
+These errors are specific to ehrQL,
+rather than Python.
+
+### Forgetting to set a population
+
+#### Failing dataset definition :x:
+
+```python
+from ehrql import create_dataset
+from ehrql.tables.core import patients
+
+dataset = create_dataset()
+dataset.age = patients.age_on("2023-01-01")
+```
+
+Run the dataset definition with:
+```
+opensafely exec ehrql:v1 generate-dataset analysis/dataset_definition.py
+```
+
+#### Error
+
+```
+A population has not been defined; define one with define_population()
+```
+
+#### Fixed dataset definition :heavy_check_mark:
+
+```python
+from ehrql import create_dataset
+from ehrql.tables.core import patients
+
+dataset = create_dataset()
+dataset.age = patients.age_on("2023-01-01")
+dataset.define_population(dataset.age > 16) # Here we have now defined a population for the dataset.
+```
+
+### Invalid feature name: `population` is a reserved name
+
+There are a few constraints on feature names in ehrQL.
+
+#### Failing dataset definition :x:
+
+```python
+from ehrql import create_dataset
+from ehrql.tables.core import patients
+
+dataset = create_dataset()
+dataset.population = patients.age_on("2023-01-01") > 16
+```
+
+Run the dataset definition with:
+```
+opensafely exec ehrql:v1 generate-dataset analysis/dataset_definition.py
+```
+
+#### Error
+
+```pytb
+Traceback (most recent call last):
+  File "/workspace/analysis/dataset_definition.py", line 6, in <module>
+    dataset.population = patients.age_on("2023-01-01") > 16
+    ^^^^^^^^^^^^^^^^^^
+AttributeError: Cannot set variable 'population'; use define_population() instead
+```
+
+#### Fixed dataset definition :heavy_check_mark:
+
+Define population with the `define_population` syntax:
+
+```python
+from ehrql import create_dataset
+from ehrql.tables.core import patients
+
+dataset = create_dataset()
+dataset.define_population(patients.age_on("2023-01-01") > 16)
+```
+
+Or rename the feature, if it is required as a separate output:
+
+```python
+from ehrql import create_dataset
+from ehrql.tables.core import patients
+
+dataset = create_dataset()
+dataset.over_16 = patients.age_on("2023-01-01") > 16
+```
+
+### Invalid feature name: `variables` is a reserved name
+
+#### Failing dataset definition :x:
+
+```python
+from ehrql import create_dataset
+from ehrql.tables.core import patients
+
+dataset = create_dataset()
+dataset.variables = patients.age_on("2023-01-01") > 16
+...
+```
+
+Run the dataset definition with:
+```
+opensafely exec ehrql:v1 generate-dataset analysis/dataset_definition.py
+```
+
+#### Error
+
+```pytb
+Traceback (most recent call last):
+  File "/workspace/analysis/dataset_definition.py", line 5, in <module>
+    dataset.variables = patients.age_on("2023-01-01") > 16
+    ^^^^^^^^^^^^^^^^^
+AttributeError: 'variables' is not an allowed variable name
+```
+#### Fixed dataset definition :heavy_check_mark:
+
+Rename the feature to something other than `variables`.
+
+```python
+from ehrql import create_dataset
+from ehrql.tables.core import patients
+
+dataset = create_dataset()
+dataset.age_greater_than_16 = patients.age_on("2023-01-01") > 16
+...
+```
+
+### Invalid feature name: feature names must not start with underscores
+
+#### Failing dataset definition :x:
+
+```python
+from ehrql import create_dataset
+from ehrql.tables.core import patients
+
+dataset = create_dataset()
+age = patients.age_on("2023-01-01")
+dataset.define_population(age > 16)
+dataset._age = age
+```
+
+Run the dataset definition with:
+```
+opensafely exec ehrql:v1 generate-dataset analysis/dataset_definition.py
+```
+
+#### Error
+
+```pytb
+Traceback (most recent call last):
+  File "/workspace/analysis/dataset_definition.py", line 7, in <module>
+    dataset._age = age
+    ^^^^^^^^^^^^^
+AttributeError: Variable names must start with a letter, and contain only alphanumeric characters and underscores (you defined a variable '_age')
+
+```
+
+#### Fixed data definition :heavy_check_mark:
+
+```python
+from ehrql import create_dataset
+from ehrql.tables.core import patients
+
+dataset = create_dataset()
+age = patients.age_on("2023-01-01")
+dataset.define_population(age > 16)
+dataset.age = age # _age feature renamed to remove the leading underscores.
+```
+
+### Re-defining a feature
+
+In the following dataset definition, `dataset.age` is first defined as `age` and then defined again as `age1`.
+
+#### Failing dataset definition :x:
+
+```python
+from ehrql import create_dataset
+from ehrql.tables.core import patients
+
+dataset = create_dataset()
+age = patients.age_on("2000-01-01")
+age1 = patients.age_on("2023-01-01")
+dataset.define_population(age > 16)
+dataset.age = age
+dataset.age = age1
+```
+
+Run the dataset definition with:
+```
+opensafely exec ehrql:v1 generate-dataset analysis/dataset_definition.py
+```
+
+#### Error
+
+```pytb
+Traceback (most recent call last):
+  File "/workspace/analysis/dataset_definition.py", line 9, in <module>
+    dataset.age = age1
+    ^^^^^^^^^^^
+AttributeError: 'age' is already set and cannot be reassigned
+```
+
+#### Fixed dataset definition :heavy_check_mark:
+
+```python
+from ehrql import create_dataset
+from ehrql.tables.core import patients
+
+dataset = create_dataset()
+age = patients.age_on("2000-01-01")
+age1 = patients.age_on("2023-01-01")
+dataset.define_population(age > 16)
+dataset.age = age
+dataset.age1 = age1 # The second age feature now has a unique name on the dataset
+```
+
+### Undefined features
+
+All features set on a dataset must be defined; in the following dataset, `age` has been
+defined on its own, but has not been defined when set on the dataset:
+
+#### Failing dataset definition :x:
+
+```python
+from ehrql import create_dataset
+from ehrql.tables.core import patients
+
+dataset = create_dataset()
+age = patients.age_on("2000-01-01")
+dataset.define_population(age > 16)
+dataset.age
+```
+
+Run the dataset definition with:
+```
+opensafely exec ehrql:v1 generate-dataset analysis/dataset_definition.py
+```
+
+#### Error
+
+```pytb
+Traceback (most recent call last):
+  File "/workspace/analysis/dataset_definition.py", line 8, in <module>
+    dataset.age
+AttributeError: Variable 'age' has not been defined
+```
+
+#### Fixed dataset definition :heavy_check_mark:
+
+```python
+from ehrql import create_dataset
+from ehrql.tables.core import patients
+
+dataset = create_dataset()
+age = patients.age_on("2000-01-01")
+dataset.define_population(age > 16)
+dataset.age = age # dataset.age is now defined
+```
+
+### Trying to set a feature that has more than one row per patient
+
+#### Failing dataset definition :x:
+
+```python
+from ehrql import create_dataset
+from ehrql.tables.tpp import practice_registrations
+
+dataset = create_dataset()
+dataset.registered_on = practice_registrations.start_date
+```
+
+The `practice_registrations` table contains multiple rows per patient.
+
+#### Error
+
+```pytb
+Traceback (most recent call last):
+  File "/workspace/analysis/dataset_definition.py", line 5, in <module>
+    dataset.registered_on = practice_registrations.start_date
+    ^^^^^^^^^^^^^^^^^^^^^
+TypeError: Invalid variable 'registered_on'. Dataset variables must return one row per patient
+```
+
+#### Fixed dataset definition :heavy_check_mark:
+
+To return the latest `registered_on` date, first sort the practice registrations table, find the
+last registration for each patient, and *then* get the start date.
+
+```python
+from ehrql import create_dataset
+from ehrql.tables.tpp import practice_registrations
+
+dataset = create_dataset()
+latest_registration_per_patient = practice_registrations.sort_by(practice_registrations.start_date).last_for_patient()
+dataset.registered_on = latest_registration_per_patient.start_date
+```
+
+### Trying to set a feature to a row rather than a value
+
+In the following dataset definition, we have reduce the practice registrations to one row per patient, but
+we have not selected a value as the feature:
+
+#### Failing dataset definition :x:
+
+```python
+from ehrql import create_dataset
+from ehrql.tables.tpp import practice_registrations
+
+dataset = create_dataset()
+dataset.registered_on = practice_registrations.sort_by(practice_registrations.start_date).last_for_patient()
+```
+
+#### Error
+
+```pytb
+Traceback (most recent call last):
+  File "/workspace/analysis/dataset_definition.py", line 5, in <module>
+    dataset.registered_on = practice_registrations.sort_by(practice_registrations.start_date).last_for_patient()
+    ^^^^^^^^^^^^^^^^^^^^^
+TypeError: Invalid variable 'registered_on'. Dataset variables must be values not whole rows
+```
+
+Fix the dataset definition by setting the feature to a single value, in this case, `start_date`.
+
+#### Fixed dataset definition :heavy_check_mark:
+
+```python
+from ehrql import create_dataset
+from ehrql.tables.tpp import practice_registrations
+
+dataset = create_dataset()
+latest_registration_per_patient = practice_registrations.sort_by(practice_registrations.start_date).last_for_patient()
+dataset.registered_on = latest_registration_per_patient.start_date
+```
+
+### Type errors in ehrQL expressions
+
+Many ehrQL comparisons require the elements being compared to be of the same type.
+
+In the following dataset definition, `age` is an integer, but in the last line we
+try to define the population by comparing age to the string `"10"`
+
+#### Failing dataset definition :x:
+
+```python
+from ehrql import create_dataset
+from ehrql.tables.core import patients
+
+dataset = create_dataset()
+age = patients.age_on("2023-01-01")
+dataset.define_population(age >= "10")
+```
+
+#### Error
+
+```pytb
+Traceback (most recent call last):
+  File "/workspace/analysis/dataset_definition.py", line 6, in <module>
+    dataset.define_population(age >= "10")
+                              ^^^^^^^^^^^
+ehrql.query_model.nodes.TypeValidationError: GE.rhs requires 'ehrql.query_model.nodes.Series[int]' but got 'ehrql.query_model.nodes.Series[str]'
+```
+
+#### Fixed dataset definition :heavy_check_mark:
+
+```python
+from ehrql import create_dataset
+from ehrql.tables.core import patients
+
+dataset = create_dataset()
+age = patients.age_on("2023-01-01")
+dataset.define_population(age >= 10)  # age is now being compared to the integer 10
+```
+
+### Invalid keywords "and", "or", "not"
+
+In normal Python, logical operations can be performed using the keywords `and`, `or` and `not`. In ehrQL
+these are prohibited and will raise an error.
+
+#### Failing dataset definition :x:
+
+```python
+from ehrql import create_dataset
+from ehrql.tables.core import patients
+
+dataset = create_dataset()
+age = patients.age_on("2023-01-01")
+dataset.define_population((age >= 16) and (age <= 80))
+```
+
+#### Error
+
+```pytb
+Traceback (most recent call last):
+  File "/workspace/analysis/dataset_definition.py", line 6, in <module>
+    dataset.define_population((age >= 16) and (age <= 80))
+                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+TypeError: The keywords 'and', 'or', and 'not' cannot be used with ehrQL, please use the operators '&', '|' and '~' instead.
+(You will also see this error if you try use a chained comparison, such as 'a < b < c'.)
+```
+
+#### Fixed dataset definition :heavy_check_mark:
+
+As described in the error message, use the operator `&` instead:
+
+```python
+from ehrql import create_dataset
+from ehrql.tables.core import patients
+
+dataset = create_dataset()
+age = patients.age_on("2023-01-01")
+dataset.define_population((age >= 16) & (age <= 80))
+```
+
+### Chaining comparisons
+
+Chained comparisons are not allowed in ehrQL.
+
+#### Failing dataset definition :x:
+
+```python
+from ehrql import create_dataset
+from ehrql.tables.core import patients
+
+dataset = create_dataset()
+age = patients.age_on("2023-01-01")
+dataset.define_population(16 < age <= 80)
+```
+
+#### Error
+
+```pytb
+Traceback (most recent call last):
+  File "/workspace/analysis/dataset_definition.py", line 6, in <module>
+    dataset.define_population(16 < age <= 80)
+                              ^^^^^^^^^^^^^^
+TypeError: The keywords 'and', 'or', and 'not' cannot be used with ehrQL, please use the operators '&', '|' and '~' instead.
+(You will also see this error if you try use a chained comparison, such as 'a < b < c'.)
+```
+
+#### Fixed dataset definition :heavy_check_mark:
+
+```python
+from ehrql import create_dataset
+from ehrql.tables.core import patients
+
+dataset = create_dataset()
+age = patients.age_on("2023-01-01")
+dataset.define_population((age >= 16) & (age <= 80))
+```
+
+### Trying to perform arithmetic operations with an integer column and a float constant
+
+In the following dataset, `age` is an integer. We cannot subtract a float from it.
+
+#### Failing dataset definition :x:
+
+```python
+from ehrql import create_dataset
+from ehrql.tables.core import patients
+
+dataset = create_dataset()
+age = patients.age_on("2023-01-01")
+dataset.age_minus_5 = age - 5.5
+```
+
+#### Error
+
+```pytb
+Traceback (most recent call last):
+  File "/workspace/analysis/dataset_definition.py", line 6, in <module>
+    dataset.age_minus_5 = age - 5.5
+                          ~~~~^~~~~
+ehrql.query_model.nodes.TypeValidationError: Subtract.rhs requires 'ehrql.query_model.nodes.Series[int]' but got 'ehrql.query_model.nodes.Series[float]'
+```
+
+#### Fixed dataset definition :heavy_check_mark:
+
+```python
+from ehrql import create_dataset
+from ehrql.tables.core import patients
+
+dataset = create_dataset()
+age = patients.age_on("2023-01-01")
+dataset.age_minus_5 = age - 5
+```
+
+### Calculate a date difference without specifying return units
+
+#### Failing dataset definition :x:
+
+```python
+from ehrql import create_dataset
+from ehrql.tables.core import patients
+
+dataset = create_dataset()
+dataset.age_in_may = "2023-05-01" - patients.date_of_birth
+```
+
+#### Error
+
+```pytb
+Traceback (most recent call last):
+  File "/workspace/analysis/dataset_definition.py", line 5, in <module>
+    dataset.age_in_may = "2023-05-01" - patients.date_of_birth
+    ^^^^^^^^^^^^^^^^^^
+TypeError: Invalid variable 'age_in_may'. Dataset variables must be values not whole rows
+```
+
+To fix this error, specify the units of the date difference that you want in the feature:
+
+#### Fixed dataset definition :heavy_check_mark:
+
+```python
+from ehrql import create_dataset
+from ehrql.tables.core import patients
+
+dataset = create_dataset()
+dataset.age_in_may = ("2023-05-01" - patients.date_of_birth).years
+```
+
+### Trying to subtract/add constants to dates
+
+#### Failing dataset definition :x:
+
+```python
+from ehrql import create_dataset
+from ehrql.tables.core import patients
+
+dataset = create_dataset()
+dataset.date_at_age_16 = patients.date_of_birth + 16
+```
+
+#### Error
+
+```pytb
+Traceback (most recent call last):
+  File "/workspace/analysis/dataset_definition.py", line 5, in <module>
+    dataset.date_at_age_16 = patients.date_of_birth + 16
+                             ~~~~~~~~~~~~~~~~~~~~~~~^~~~
+TypeError: unsupported operand type(s) for +: 'DatePatientSeries' and 'int'
+```
+
+ehrQL cannot add an integer to a date - it needs to know what sort of time unit
+we are adding (days, months, years).
+
+#### Fixed dataset definition :heavy_check_mark:
+
+```python
+from ehrql import create_dataset, years
+from ehrql.tables.core import patients
+
+dataset = create_dataset()
+dataset.date_at_age_16 = patients.date_of_birth + years(16)
+```
+
+### Incorrectly referencing a table column
+
+#### Failing dataset definition :x:
+
+```python
+from ehrql import create_dataset
+from ehrql.tables.core import clinical_events
+
+dataset = create_dataset()
+first_event = clinical_events.sort_by(date).first_for_patient()
+dataset.event_date = first_event.date
+```
+
+#### Error
+
+```pytb
+Traceback (most recent call last):
+  File "/workspace/analysis/dataset_definition.py", line 5, in <module>
+    first_event = clinical_events.sort_by(date).first_for_patient()
+                                          ^^^^
+NameError: name 'date' is not defined
+```
+
+#### Fixed dataset definition :heavy_check_mark:
+
+Columns must be specified as the table attribute:
+
+```python
+from ehrql import create_dataset
+from ehrql.tables.core import clinical_events
+
+dataset = create_dataset()
+first_event = clinical_events.sort_by(clinical_events.date).first_for_patient()
+dataset.event_date = first_event.date
+```
+
+### Specifying a default for `case` which is a different type to the values
+
+In the following dataset definition, two age groups are defined as integers (1 and 2). A default
+value (for patients who don't fall into one of the categories) is defined as "unknown".  This is
+an error - any default value given for a case statement must be of the same type (or None).
+
+#### Failing dataset definition :x:
+
+```python
+from ehrql import create_dataset, case, when
+from ehrql.tables.core import patients
+
+dataset = create_dataset()
+
+age = patients.age_on("2023-01-01")
+dataset.age_group = case(
+   when(age < 10).then(1),
+   when(age > 80).then(2),
+   otherwise="unknown",
+)
+```
+
+#### Error
+
+```pytb
+Traceback (most recent call last):
+  File "/workspace/analysis/dataset_definition.py", line 7, in <module>
+    dataset.age_group5 = case(
+                         ^^^^^
+ehrql.query_model.nodes.TypeValidationError: Case.default requires 'ehrql.query_model.nodes.Series[int] | None' but got 'ehrql.query_model.nodes.Series[str]'
+```
+
+#### Fixed dataset definition :heavy_check_mark:
+
+```python
+from ehrql import create_dataset, case, when
+from ehrql.tables.core import patients
+
+dataset = create_dataset()
+
+age = patients.age_on("2023-01-01")
+dataset.age_group = case(
+   when(age < 10).then(1),
+   when(age > 80).then(2),
+   otherwise=0,
+)
+```
+
+### Using `is_in` without a container
+
+#### Failing dataset definition :x:
+
+```python
+from ehrql import create_dataset
+from ehrql.tables.core import patients
+
+dataset = create_dataset()
+
+age = patients.age_on("2023-01-01")
+dataset.age_30 = age.is_in(30)
+```
+
+#### Error
+
+```pytb
+Traceback (most recent call last):
+  File "/workspace/analysis/dataset_definition.py", line 7, in <module>
+    dataset.age_30 = age.is_in(30)
+                     ^^^^^^^^^^^^^
+ehrql.query_model.nodes.TypeValidationError: In.rhs requires 'ehrql.query_model.nodes.Series[collections.abc.Set[int]]' but got 'ehrql.query_model.nodes.Series[int]'
+```
+
+:notepad_spiral: This is also an error:
+
+```python
+dataset.age_30_or_40 = age.is_in(30, 40)
+```
+
+#### Fixed dataset definition :heavy_check_mark:
+
+Arguments passed to `is_in` must be wrapped in a python container - a set, list or tuple.
+All of the following features defined with `is_in` are valid.
+
+```python
+from ehrql import create_dataset
+from ehrql.tables.core import patients
+
+dataset = create_dataset()
+
+age = patients.age_on("2023-01-01")
+dataset.age_30_list = age.is_in([30])
+dataset.age_30_or_40_set = age.is_in({30, 40})
+dataset.age_30_or_40_tuple = age.is_in((30, 40))
+```