#4330 'diff(CONST,VAR) should simplify to zero

[Stavros Macrakis]

Currently:

'diff(2/3,x) => 0      <<< good
'diff(%pi,x) => 0      <<< good
'diff(%pi/2,x) => 'diff(%pi/2,x)    <<< why??

It looks like simp.lisp/simpderiv should use $constantp rather than the long conditional it currently uses.
simpderive should also give an error if the variable is a constant:

diff(2/3,%pi) => ERROR      <<< good
'diff(2/3,%pi) => 0         <<< should be ERROR
'diff(%pi,%pi) => 1         <<< should be ERROR

[Robert Dodier]

I don't see why Maxima should prohibit differentiating with respect to a constant. Surely it's equivalent to just renaming the variable, carrying out the operation, and naming it back again.