theorem
  for C,e,M,G st C.e = 1 holds Polish-ext-head(Polish-unOp(C,M,e).G) = e
proof
  let C,e,M,G;
  set K = dom C;
  set F = Polish-unOp(C,M,e).G;
  assume C.e = 1;
  then F = e^G by Def15;
  then F is K-headed & K-head F = e;
  hence thesis by Th12;
end;
