theorem Th50:
  for A,B,C be set, f be Function st A <> {} & B <> {} & f in
  Funcs(A,Funcs(B,C)) holds commute f in Funcs(B,Funcs(A,C))
proof
  let A,B,C be set, f be Function;
  assume A <> {} & B <> {} & f in Funcs(A,Funcs(B,C));
  then [:A,B:] <> {} & uncurry f in Funcs([:A,B:],C) by Th11,ZFMISC_1:90;
  hence thesis by Th10;
end;
