richer extraction syntax

lib/resource/exper/abstract/Relative.gf

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+--1 Relative clauses and pronouns
+
+abstract Relative = Cat ** {
+
+  fun
+
+-- The simplest way to form a relative clause is from a clause by
+-- a pronoun similar to "such that".
+
+    RelCl    : Cl -> RCl ;            -- such that John loves her
+
+-- The more proper ways are from a verb phrase 
+-- (formed in [``Verb`` Verb.html]) or a sentence 
+-- with a missing noun phrase (formed in [``Sentence`` Sentence.html]).
+
+    RelVP    : RP -> VP -> RCl ;      -- who loves John
+    RelSlash : RP -> Slash -> RCl ;   -- whom John loves
+
+-- Relative pronouns are formed from an 'identity element' by prefixing
+-- or suffixing (depending on language) prepositional phrases.
+
+    IdRP  : RP ;                      -- which
+    FunRP : Prep -> NP -> RP -> RP ;  -- all the roots of which 
+
+}
+