
theorem Th7:
  for f being FinSequence, k1,k2 being Nat st k1>k2 holds
  smid(f,k1,k2)={}
proof
  let f be FinSequence,k1,k2 be Nat;
  assume
A1: k1>k2;
  set g=(f/^(k1-'1));
  k2+1<=k1 by A1,NAT_1:13;
  then smid(f,k1,k2)=g|0 by NAT_2:8;
  hence thesis;
end;
