theorem Th20:
  l = [[dom l,cod l],l`2]
proof
  consider g,C1,C2 such that
A1: l = [[C1,C2],g] and
  union C2 = {} implies union C1 = {} and
  g is Function of union C1,union C2 and
  for x,y st {x,y} in C1 holds {g.x,g.y} in C2 by Th18;
  thus thesis by A1;
end;
