Ignoring the bits about “real world”, as we often do, and speaking only to the structural description, I’ll add a few points (maybe more than you’re asking for):
- We didn’t use to give everything (except quantifiers) an intrinsic argument (ARG0), but now it is assumed for a well-formed structure; they serve as imperfect identifiers of EPs
- The intrinsic variable property (IVP; see below) essentially states this requirement and it is (mostly) necessary for conversion to dependencies (DMRS or EDS)
- But not all grammars (maybe not even the ERG) have been fully updated to conform to the IVP
- All
xvariables are supposed to have a quantifier, but you may find some (even in the ERG) that don’t; these are generally considered grammar bugs, but there are discussions of relaxing this for dropped arguments (see below) - Dependency representations (EDS and DMRS) do not need (or have) variables because nodes (formerly EPs) have unique identifiers
Intrinsic Variable Property
Oepen and Lønning, 2006 explicitly assumed the presence of a distinguished variable for conversion of MRS to EDS dependencies, and Copestake, 2009 defined the similar characteristic variable property for converting to DMRS dependencies. The distinguished and characteristic variables are historical terms that refer to the same thing (ARG0s), and we later agreed to call them intrinsic variables. In any case, the general idea is that each EP gets an ARG0. Somewhere I’ve described the differences in how Copestake, 2009 defined the (rebranded) “intrinsic variable property” and how I evaluate it in PyDelphin, but I can’t turn up anything at this moment. Basically there are three conditions under discussion:
- All non-quantifier EPs have an ARG0
- All variables are the ARG0 of some EP
- The ARG0 of an EP is unique to it (not the ARG0 of any other EP)
Copestake, 2009 defines the IVP as (3), which, in her description, implies (2). She further assumes (1) for the purposes of the paper. PyDelphin defines the IVP as (1) and (3). It does not require (2) because of observed (and presumed good) MRSs that do not follow (2), but this depends on what a “variable” is. Handles (h), for instance, are generally not considered variables, despite h being in the variable hierarchy. It’s less clear if underspecified variables i, p, and u fall under (2), and they (particularly i) appear in a few contexts (dropped arguments, scopal modifiers, and true underspecification where it’s not decidable or decided if an EP represents an instance or eventuality); for these see below.
Dropped arguments, etc.
A convention in MRS is that when an argument is dropped (e.g., I left. vs I left Oregon.) there is no EP introduced for the missing argument and its variable becomes underspecified (an i instead of an x). This is, I guess, to avoid requiring a quantifier for the variable. However i variables are also used for some scopal modifiers (e.g., for _never_a_1 in I never left.) and things like compositional numbers (for the EPs for “20” and “8” in Kim drove for 28 hours). I brought up this issue at our last research summit in Cambridge (see Section 3 of my presentation notes and the minutes of the discussion). There were some proposals for resolving the various meanings of i variables in MRSs but I think the current situation still stands.
Other links
You might find some more information in these places:
- PyDelphin’s documentation of the
delphin.variablemodule - Sections 2.5 and 2.6 of my dissertation (I don’t think it has any new information not already discussed but you might find other parts of that chapter informative).