Returns true if the expression f is an indexed function in the name(s) x, that is, if the input is a function and the variable(s) x are used as the index variable in a derivative, limit, integral, sum, product, and transform (for example, fourier and laplace). The purpose of this routine is to avoid substitution for x in an expression like diff(f(x), x) or int(f(x), x=a..b) where substitution of  would produce an erroneous expression, for example.