I have committed extensions to sdiffgrad to allow the value NIL and a
lambda expression on the property list for the definition of the
derivatives of a function.
These extensions do not changes the old behavior of Maxima.
The value NIL can be used, if a derivative is not known. The function
sdiffgrad generates for this case the noun form. This resolves a problem
with the derivative of functions with more then one argument. See e.g.
the functions li[n](x) and psi[n](x) and the related bug report ID:
2699862 "derivative of polylogarithm".
The lambda expression on the property list allows to evaluate a
derivative which depends e.g. on the sign of an argument. This is
implemented for the function gamma_incomplete. See bug report ID:
2793294 "derivative of gamma_incomplete".
Dieter Kaiser