took tense linearizations apart from Common/Cat to Tense, for bettter structure of lib

lib/src/abstract/Common.gf

@@ -3,14 +3,6 @@
 -- This module defines the categories that uniformly have the linearization
 -- ${s : Str}$ in all languages. 
 
--- Moreover, this module defines the abstract parameters of tense, polarity, and
--- anteriority, which are used in [``Phrase`` Phrase.html] to generate different
--- forms of sentences. Together they give 4 x 2 x 2 = 16 sentence forms.
-
--- These tenses are defined for all languages in the library. More tenses
--- can be defined in the language extensions, e.g. the "passé simple" of
--- Romance languages in [``ExtraRomance`` ../romance/ExtraRomance.gf].
-
 abstract Common = {
 
   cat
@@ -51,14 +43,4 @@ abstract Common = {
     Pol ;   -- polarity                            e.g. positive, negative
     Ant ;   -- anteriority                         e.g. simultaneous, anterior
 
-  fun
-    TTAnt : Tense -> Ant -> Temp ;
-
-    PPos, PNeg : Pol ;           -- I sleep/don't sleep
-
-    TPres  : Tense ;                
-    ASimul : Ant ;
-    TPast, TFut, TCond : Tense ; -- I slept/will sleep/would sleep --# notpresent
-    AAnter : Ant ;               -- I have slept                   --# notpresent
-
 }

lib/src/abstract/Grammar.gf

@@ -18,6 +18,6 @@ abstract Grammar =
   Phrase,
   Text,
   Structural,
-  Idiom ;
-
+  Idiom,
+  Tense ;
 

lib/src/abstract/Tense.gf

@@ -0,0 +1,22 @@
+--1 Common: Structures with Common Implementations.
+
+-- This module defines the abstract parameters of tense, polarity, and
+-- anteriority, which are used in [``Phrase`` Phrase.html] to generate different
+-- forms of sentences. Together they give 4 x 2 x 2 = 16 sentence forms.
+
+-- These tenses are defined for all languages in the library. More tenses
+-- can be defined in the language extensions, e.g. the "passé simple" of
+-- Romance languages in [``ExtraRomance`` ../romance/ExtraRomance.gf].
+
+abstract Tense = Common ** {
+
+  fun
+    TTAnt : Tense -> Ant -> Temp ;
+
+    PPos, PNeg : Pol ;           -- I sleep/don't sleep
+
+    TPres  : Tense ;                
+    ASimul : Ant ;
+    TPast, TFut, TCond : Tense ; -- I slept/will sleep/would sleep --# notpresent
+    AAnter : Ant ;               -- I have slept                   --# notpresent
+}