It makes definition of principal constraint clearer:
principal constraint is a constraint that has at least one arg
of type node<> and passes to it ruleOrigin in at least one rule.
Such constraints are most likely meant to define relations
of nodes to terms, e.g. `typeOf` -- which is what we're interested in.
At the same time semantics of values inside terms is less defined.
TermFeatures are also untyped, and so complete analysis of
ValueFeatures would require full code traversal of coderules program
to find out whether a value feature is used to carry smth of `node` type.
Relevant to MPSCR-62.
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