restored multimodal and mathematical sources

lib/resource-1.0/multimodal/Multimodal.gf

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+abstract Multimodal =
+  Noun,
+--  Verb, 
+  Adjective,
+  Adverb,
+  Numeral,
+--  Sentence, 
+--  Question,
+--  Relative,
+--  Conjunction,
+--  Phrase,
+--  Tensed,
+  Structural,
+  Demonstrative,
+  Lexicon
+  ** {} ;