theorem Th18:
  W.(a,p) = x & W.(a,b) = v implies (*'(a,p) = b iff Phi(a,x) = v)
proof
  assume that
A1: W.(a,p) = x and
A2: W.(a,b) = v;
  Phi(a,x) = W.(a,*'(a,p)) by A1,Th15;
  hence thesis by A2,MIDSP_2:32;
end;
