theorem Th11:
  (not F in W) & W1 = W \/ {F} implies len(W1) = len(W) + len F
proof
  assume
A1: ( not F in W)& W1 = W \/ {F};
  consider L such that
A2: rng L = Subformulae H & L is one-to-one by FINSEQ_4:58;
  len(W1) = len(L,W1) by A2,Def26
    .= len(L,W) + len F by A1,A2,Th7;
  hence thesis by A2,Def26;
end;
