reserve a,b,c for set;

theorem Th26:
  for D being non empty set,r,CR being File of D st CR
  is_postposition_of r holds 0<=len r-len CR
proof
  let D be non empty set,f,g be File of D;
  assume g is_postposition_of f;
  then len g <= len f by FINSEQ_8:23;
  then len g - len g <= len f - len g by XREAL_1:9;
  hence thesis;
end;
