InnerDifferentiate​(AngouriMath.​Entity.​Variable)

A name that cannot vary contributes nothing to any derivative, so
d(anything)/d(pi) is 0 and not 1 even where the name
matches, which is what
#993 is
about. The test is whether the name evaluates to a number, not whether it is
spelled like a constant: a name a binder declares can vary even when it is
spelled pi, and it evaluates to itself.

And where it does vary, the comparison is of the node, not its name. A
Constant and a Variable can share a spelling — that
is what a binder over pi produces — and comparing the spelling made
derivative(arccos(0) * pi, pi) differentiate the pi / 2 that
arccos(0) simplifies to as though it were the index.
#984