Function: forstep
Section: programming/control
C-Name: forstep
Prototype: vV=GGGI
Help: forstep(X=a,b,s,seq): the sequence is evaluated, X going from a to b
 in steps of s (can be a vector of steps). If b is set to +oo the loop will
 not stop.
Doc: evaluates \var{seq},
 where the formal variable $X$ goes from $a$ to $b$, in increments of $s$.
 Nothing is done if $s>0$ and $a>b$ or if $s<0$ and $a<b$. $s$ must be in
 $\R^*$ or a vector of steps $[s_1,\dots,s_n]$. In the latter case, the
 successive steps are used in the order they appear in $s$.

 \bprog
 ? forstep(x=5, 20, [2,4], print(x))
 5
 7
 11
 13
 17
 19
 @eprog\noindent Setting $b$ to \kbd{+oo} will start an infinite loop; it is
 expected that the caller will break out of the loop itself at some point,
 using \kbd{break} or \kbd{return}.
