Function: ffcompomap
Section: number_theoretical
C-Name: ffcompomap
Prototype: GG
Help: ffcompomap(f, g): Let k, l, m be three finite fields and f a (partial) map
 from l to m and g a partial map from k to l, return the (partial) map f o g
 from k to m.
Doc: Let $k$, $l$, $m$ be three finite fields and $f$ a (partial) map from $l$
 to $m$ and $g$ a (partial) map from $k$ to $l$, return the (partial) map $f
 \circ g$ from $k$ to $m$.
 \bprog
 a = ffgen([3,5],'a); b = ffgen([3,10],'b); c = ffgen([3,20],'c);
 m = ffembed(a, b); n = ffembed(b, c);
 rm = ffinvmap(m); rn = ffinvmap(n);
 nm = ffcompomap(n,m);
 ffmap(n,ffmap(m,a)) == ffmap(nm, a)
 %5 = 1
 ffcompomap(rm, rn) == ffinvmap(nm)
 %6 = 1
 @eprog
