15 lines (9 with data), 295 Bytes

AnyVar l0 = lr/(er+1);
AnyVar eps0 = 0.03;
AnyVar eps = (meas.Pos[0]-l0)/l0;


AnyFloat Val = {
                iffun(gteqfun(eps,0.0),
                iffun(gtfun(eps,2*eps0),-k*(eps-eps0),-k*1/4*(eps^2/eps0)),
                0.0)
               };

AnyVar EqStiffness = k/l0;