
theorem Th23:
  for D being non empty set,f,g being FinSequence of D
  st g is_postposition_of f holds len g <=len f
proof
  let D be non empty set,f,g be FinSequence of D;
  assume a1: g is_postposition_of f;
A2: len (Rev g)=len g by FINSEQ_5:def 3;
  len (Rev f)=len f by FINSEQ_5:def 3;
  hence thesis by a1,A2,FINSEQ_1:63;
end;
