(Extend) Rename WhatSSC to EmbedSSlash and change type to SSlash -> SC

src/abstract/Extend.gf

@@ -148,9 +148,11 @@ abstract Extend = Cat ** {
 
     PredIAdvVP : IAdv -> VP -> QCl ; -- how to walk?
 
--- alternative SC, with "what"
+-- alternative to EmbedQS. For English, EmbedQS happens to work,
+-- because "what" introduces question and relative. The default linearization
+-- could be e.g. "the thing we did (was fun)".
 
-    WhatSSC : S -> SC  ;   -- what we did (was fun)
+    EmbedSSlash : SSlash -> SC  ;   -- what we did (was fun)
 
 -- reflexive noun phrases: a generalization of Verb.ReflVP, which covers just reflexive pronouns
 -- This is necessary in languages like Swedish, which have special reflexive possessives.