theorem Th54:
  p1 <> p2 implies <*p1,p2,p3*>:-p2 = <*p2,p3*>
proof
  assume
A1: p1 <> p2;
  p2 in { p1,p2,p3 } by ENUMSET1:def 1;
  then p2 in rng<*p1,p2,p3*> by Lm2;
  hence <*p1,p2,p3*>:-p2 = <*p2*>^(<*p1,p2,p3*> |-- p2) by Th41
    .= <*p2,p3*> by A1,Th30;
end;
