Note [The substitution in MaybeApart]

The constructor MaybeApart carries data with it, typically a TvSubstEnv. Why?
Because consider unifying these:

(a, a, Int) ~ (b, [b], Bool)

If we go left-to-right, we start with [a |-> b]. Then, on the middle terms, we
apply the subst we have so far and discover that we need [b |-> [b]]. Because
this fails the occurs check, we say that the types are MaybeApart (see above
Note [Infinitary substitutions]). But, we can't stop there! Because if we
continue, we discover that Int is SurelyApart from Bool, and therefore the
types are apart. This has practical consequences for the ability for closed
type family applications to reduce. See test case
indexed-types/should_compile/Overlap14.