Multiple answers

[phenomist]

If #26 is supported, multiple answers shouldn't be too much additional UI burden.

The simplest behaviors to add (and probably the most common usecases) are "solve if any one answer out of a set is submitted" (e.g. alternative answers) and "solve if all answers out of a set are submitted". This can probably be generalized to "solve if M out of N answers are submitted" easily as well, though a fully flexible approach might want to do something pertaining to multiple point currencies (#23).