Altered comment.
Deleted unnecessary wrapper around mx/zero-{matrix,vector}
Fixed bug in worth-saying-ids.

:when class of for had a # in front of parens.  but what's
after :when is not supposed to be a function; it's just an expression,
a call/application.  So by putting a # there, I was returning a
function, which was always truthy.  Got rid of the #.