Ordering items in index

An unexpected (to me!) index ordering in memoir. Here's a MWE.

\documentclass[11pt]{memoir}\usepackage{makeidx}\makeindex\begin{document}Aristotle \index{Aristotle}(CP)      \index{:@(CP)}$\to$     \index{0@$\to$}\printindex\end{document}

Since : comes in ascii between0 and A I would expect the index to be ordered

->, 1(CP), 1Aristotle, 1

But I get the unwanted

(CP), 1->, 1Aristotle, 1

Query: how to force what I want -- index listing symbols first (gap) list of rules of inference (gap) main alphabetic sequence.