Motivation is that implicit (non-overriden) list features must unify
with list of any length. Consider an example:
`classifier(cls: node-ptr/A/) = classifier(cls: node-ptr/A/ param: [B,C])`
With previous approach it would fail, which seems counter-intuitive to me.
So, if empty list (e.g. of type parameters) is meant, then it must be
specified explicitly.
There's one thing to remember, though: such wildcard list features
won't match with MetaLogicalArrays in rule heads.
(e.g. see dpromote_* rules and consider what happens when
a classifier with unbound `parameter` feature comes in)
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