don't backtrack

[gelisam]

In #23, we introduced a new algorithm which tries one type substitution, then backtracks and tries another substitution if it doesn't work. Backtracking is scary, because it could end up doing an exponential amount of work. And in this case it doesn't seem like the backtracking is needed, because we have a Fun patternF _ we need to pattern-match on, so we don't need to try many possibilities, we only have to try the ones which can potentially get us something of the form Fun patternF _.