Function: ispolygonal
Section: number_theoretical
C-Name: ispolygonal
Prototype: lGGD&
Help: ispolygonal(x,s,{&N}): true(1) if x is an s-gonal number, false(0) if
 not (s > 2). If N is given set it to n if x is the n-th s-gonal number.
Doc: true (1) if the integer $x$ is an s-gonal number, false (0) if not.
 The parameter $s > 2$ must be a \typ{INT}. If $N$ is given, set it to $n$
 if $x$ is the $n$-th $s$-gonal number.
 \bprog
 ? ispolygonal(36, 3, &N)
 %1 = 1
 ? N
 @eprog
