theorem
  for A,B,C being set st Funcs(A,B) <> {} & Funcs(B,C) <> {} holds
  Funcs(A,C) <> {}
proof
  let A,B,C be set such that
A1: Funcs(A,B) <> {} and
A2: Funcs(B,C) <> {};
  consider g being object such that
A3: g in Funcs(B,C) by A2,XBOOLE_0:def 1;
  ex f being object st f in Funcs(A,B) by A1,XBOOLE_0:def 1;
  hence thesis by A3,Th127;
end;
