check if message is in a particular sequence?

What's the fastest/easiest way to check if a particular message
is a member of a particular sequence?

I thought I'd be able to compare the message number against the output
of "mark -list", but since sequences can be abbreviated with range
notation, that gets complicated quickly.  For example, message 6 is in
the sequence todo, but it's hard to tell from this:
    $ mark -list -sequence todo
    todo: 1 3 5-8 13 16 46 49 52-53

paul
=----------------------
 paul fox, pgf(_at_)foxharp(_dot_)boston(_dot_)ma(_dot_)us (arlington, ma, 
where it's 47.3 degrees)