Function: arity
Section: programming/specific
C-Name: arity0
Prototype: G
Help: arity(C): return the arity of the closure C.
Doc: return the arity of the closure $C$, i.e., the number of its arguments.
 \bprog
 ? f1(x,y=0)=x+y;
 ? arity(f1)
 %1 = 2
 ? f2(t,s[..])=print(t,":",s);
 ? arity(f2)
 %2 = 2
 @eprog\noindent Note that a variadic argument, such as $s$ in \kbd{f2} above,
 is counted as a single argument.
