Error message for Mt could be more helpful

[gsinclair]

Given the code

Mt tl1.description, /Overview of number systems/

I ran the test and got the error

Expect a String and a Regexp (any order)

The error is helpful if you genuinely misuse the Mt test, but in this case I thought I was giving it a string and a regex. It must be that tl1.description is nil. So a more helpful error message would be

Expect a String and a Regexp (any order); got [nil, Regexp]

Another thing to consider is this: should Mt accept nil and a regex, and fail the test (because no match was found) rather than error the test (because the wrong argument types were given)? This could be argued either way and requires further thought.