theorem Th10:
  F in W implies len(W\{F}) = len(W) - len F
proof
  assume
A1: F in W;
  consider L such that
A2: rng L = Subformulae H & L is one-to-one by FINSEQ_4:58;
  len(W\{F}) = len(L,W\{F}) by A2,Def26
    .= len(L,W) - len F by A1,A2,Th6;
  hence thesis by A2,Def26;
end;
