
theorem Th4:
  for f being FinSequence,k1,k2 being Nat st k1<=k2 holds
    smid(f,k1,k2) = mid(f,k1,k2)
proof
  let f be FinSequence,k1,k2 be Nat;
  assume
A1: k1<=k2;
  then k2-'k1+1=k2-k1+1 by XREAL_1:233
    .=k2+1-k1
    .=k2+1-'k1 by A1,NAT_1:12,XREAL_1:233;
  hence thesis by A1,FINSEQ_6:def 3;
end;
