theorem
  for f1 being non empty FinSequence of D, f2 being FinSequence of D
  holds (f1^'f2)/.1 = f1/.1
proof
  let f1 be non empty FinSequence of D, f2 be FinSequence of D;
A1: 1 in dom f1 by FINSEQ_5:6;
  1 in dom (f1^(2, len f2)-cut f2) by FINSEQ_5:6;
  hence (f1^'f2)/.1 = (f1^(2, len f2)-cut f2).1 by PARTFUN1:def 6
    .= f1.1 by A1,FINSEQ_1:def 7
    .= f1/.1 by A1,PARTFUN1:def 6;
end;
