Remaining synonym issues

[Anaphory]

We need better tests for non-obvious constant features (simple: all languages either ? or V; complicated: All languages either ? or V, W – the first should easily show up as constant feature because V is the only value. The second one would be constant in each sub-feature, but differentiating it from the case where even a single language has V instead of V, W will not be easy.)

Originally posted by @Anaphory in #144 (comment)