On entry: the point at which components of the solution and derivative are to be evaluated.
twant should not normally be an extrapolation point, that is
twant should satisfy
or if integration is proceeding in the negative direction
where
told is the previous integration point which is held in an element of the array
rwork and is, to within rounding,
${\mathbf{t}}-{\mathbf{hused}}$. (
hused is given by
d02lyf.) Extrapolation is permitted but not recommended, and
${\mathbf{ifail}}={\mathbf{2}}$ is returned whenever extrapolation is attempted.