theorem Th95:
  P1 c= P2 implies Y|`P1 c= Y|`P2
proof
  assume
A1: P1 c= P2;
  let x,y;
  assume [x,y] in Y|`P1;
  then [x,y] in P1 & y in Y by Def10;
  hence thesis by A1,Def10;
end;
