theorem Th23:
  W.(a,p) = x & W.(a,b) = v & Phi(x) = v implies *'(a,p) = b
proof
  assume
A1: W.(a,p) = x & W.(a,b) = v & Phi(x) = v;
  Phi(x) = Phi(a,x) by Def12;
  hence thesis by A1,Th18;
end;
