[stack] 'while' derived from 's', 'k', and 'if' in Factor

I took the 's' and 'k' combinators from here:
http://tunes.org/~iepos/joy.html

Yes, the definition for 's' could be much better, but I don't know  
really know Factor's combinators. For the Joy programmers, note that  
Factor's 'if' requires a boolean on the stack, not a quotation that  
yields a boolean.

Ah well. Back to work.

- John

- - - - - - - - - -

! The 's' combinator
! [$A] [$B] [$C] [$D] s == [[$A] $B] $D [$A] $C
: s
   [ [ over ] dip ] dip
   [ [ [ curry ] dip ] dip ] dip
   -rot
   [ [ call ] dip ] dip
   call
;

! The 'k' combinator
! [$A] [$B] k == $B
: k nip call ;

! Essential combinators:
: j [ k ] [ [ ] [ [ ] ] ] [ ] s s s ;
: qc [ ] [ ] [ ] s ;
: d [ ] k ;
: q qc d ;
: x q j ;
: i [ ] x j d ;
: c qc x i ;
: p [ ] [ ] s d ;
: m c i ;
: o [ x j i ] p p ;
: v x c [ x ] j ;
: c2 v v ;

! The 'w' ("while") combinator
! [$A] [$B] w == $B [$A [$A] [$B] w] [ ] if
! Example usage: 10 [ c . 1 - ] [ c 0 > ] w d
: w [ [ c2 ] j c p p p [ x ] j o x j [ ] if ] m ;

! Key:
! j  = dip
! qc = dup quote swap
! d  = drop
! q  = quote
! x  = swap
! i  = call (factor), apply (cat), i (joy)
! c  = copy, dup
! p  = partial-apply, curry (factor, cat), cons (joy)
! m  = mockingbird, dup i
! o  = compose
! v  = over
! c2 = 2dup
! w  = while (A [A -> A] [A -> A Bool] -> A)