theorem Th4:
  ex f,A,B st m = [[A,B],f] & (B = {} implies A = {}) & f is Function of A,B
proof
  m in { [[A,B],f]: (B={} implies A={}) & f is Function of A,B};
  then ex A,B,f st m = [[A,B],f] & (B={} implies A={}) & f is Function of A,B;
  hence thesis;
end;
