theorem Th24:
  (a,x).W = p & (a,v).W = b & *'(a,p) = b implies Phi(x) = v
proof
  assume (a,x).W = p & (a,v).W = b & *'(a,p) = b;
  then Phi(a,x) = v by MIDSP_2:33;
  hence thesis by Def12;
end;
