Here’s a very broad question I got recently, about the HPSG formalism. I am trying to understand this question better.
“…for formalisms like CFG, LCFRS, TAG, etc. […] there are characterizations of what kind of trees you can and can’t build with the grammar. For example if your LCFRS has fan-out 2, then you know that you can have discontinuous constituents with one gap, but no more than one. If your LCFRS is well-nested, you know that you can nest constituents in some ways but not others, etc. Has anyone attempted to give an abstract characterization of the set of trees that can be covered by an HPSG grammar?”
I am not sure I understand this question well enough. Could someone help me by either rephrasing it or pointing me to some specific literature?