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
$\mathbf{opt}\mathbf{\to}\mathbf{tcurr}-\mathbf{opt}\mathbf{\to}\mathbf{hlast}\le {\mathbf{twant}}\le \mathbf{opt}\mathbf{\to}\mathbf{tcurr}$.
or if integration is proceeding in the negative direction
$\mathbf{opt}\mathbf{\to}\mathbf{tcurr}-\mathbf{opt}\mathbf{\to}\mathbf{hlast}\ge {\mathbf{twant}}\ge \mathbf{opt}\mathbf{\to}\mathbf{tcurr}$.
Extrapolation is permitted but not recommended and a
fail value of
NW_EXTRAPOLATION is returned whenever extrapolation is attempted.