identified and reported by Marcel Hendrix,
expressions of this kind could trigger a segmentation violation.
PTdifferentiate() roughly evaluates to
ternary_fcn(ge0(0-expr), 0, PTdifferentate(expr))
and mkb() optimizes
0 - expr --> unary_minus(expr)
IFeval() invokes PTeval() for the derivative too,
PTeval() looks at the incorrect tree->funcnum
and tries to PTeval for a second argument which is not there,
(unary_minus does not have a second argument)
causing a segmentation fault.
rlar