Current coe of fhcom type is wrong

[favonia]

There's a very subtle substitution bug that could potentially be triggered (but none of our examples did). This will be fixed in #270.

Technical details: in the last rule on page 9 of Part III, N has an x binder that should prevent the prevailing substitution [r/x] in Q from being applied to the inner B. This is unfortunately not coded correctly, and a correct fix will disrupt the current coding style.