theorem
  for M1 be Matrix of t,k,D, M2 be Matrix of m,k,D st n in dom M2 & i =
  len M1 + n holds Line(M1^M2,i) = Line(M2,n)
proof
  let M1 be Matrix of t,k,D;
  let M2 be Matrix of m,k,D;
  assume that
A1: n in dom M2 and
A2: i = len M1 + n;
  reconsider n1=n as Element of NAT by ORDINAL1:def 12;
  i in dom (M1^M2) by A1,A2,FINSEQ_1:28;
  hence Line(M1^M2,i) = (M1^M2).i by MATRIX_0:60
    .= M2.n1 by A1,A2,FINSEQ_1:def 7
    .= Line(M2,n) by A1,MATRIX_0:60;
end;
