theorem :: Reflexivity of Big_Theta (Problem 3.18 Part 1)
  for f being eventually-nonnegative Real_Sequence holds f in Big_Theta( f)
proof
  let f be eventually-nonnegative Real_Sequence;
  f in Big_Oh(f) & f in Big_Omega(f) by Th10,Th20;
  hence thesis by XBOOLE_0:def 4;
end;
