theorem
  f is nonnegative & g is nonnegative & h is nonnegative implies f+g+h
  is nonnegative
proof
  assume that
A1: f is nonnegative & g is nonnegative and
A2: h is nonnegative;
  f+g is nonnegative by A1,Th56;
  hence thesis by A2,Th56;
end;
