
theorem Th19:
  for f being FinSequence st 1<= len f holds f is_substring_of f,1
proof
  let f be FinSequence;
  assume
A1: 1<= len f;
  1-'1+len f =0+len f by NAT_2:8
    .=len f;
  then mid(f,1,(1-'1)+len f)=f by A1,FINSEQ_6:120;
  hence thesis by A1;
end;
