theorem Th10:
  for S,T,p st S-head p in T holds p is T-headed & S-head p = T-head p
proof
  let S,T,p;
  set q = S-head p;
  set r = S-tail p;
  assume A1: q in T;
  p = q^r;
  hence thesis by A1, POLNOT_1:52;
end;
