Floating.Polytype kind = | Never_add_mangler| Always_add_mangler| Add_mangler_if_more_than_one_eltBehaves like Always_add_mangler if the set on the RHS of the binding, when evaluated with Expand_atoms_bound_to_sets, has more than one element, and like Never_add_mangler otherwise.
For Add_mangler_if_more_than_one_elt specifically, we only permit exactly one (_, singleton) Expression.t on the RHS. Loosely speaking, if the expression on the RHS contains a Union, it is probably true that it always evaluates to a set with more than one element, and the Always_add_mangler version of the attribute should be used instead.