theorem Th67:
  for a,b,c,d being object st a <> c holds (a,c) --> (b,d) = { [a,b],
  [c,d] }
proof
  let a,b,c,d be object such that
A1: a <> c;
  set f = {a} --> b, g = {c} --> d;
  {a} --> b = {[a,b]} & {c} --> d = {[c,d]} by ZFMISC_1:29;
  hence (a,c) --> (b,d) = {[a,b]} \/ {[c,d]} by A1,Th31,ZFMISC_1:11
    .= { [a,b], [c,d] } by ENUMSET1:1;
end;
