invariable verb must_VV in persian

src/persian/ParadigmsPes.gf

@@ -122,7 +122,9 @@ oper
    compoundV : Str -> V -> V = \s,v -> {s = \\vf => s ++ v.s ! vf ; lock_V = <>} ;     
    compoundV : Str -> V2 -> V = \s,v -> {s = \\vf => s ++ v.s ! vf ; lock_V = <>} ;
    };
- 
+
+   invarV : Str -> V -- for verbs like  " بایستن " ("must"), which don't inflect
+     = \s -> lin V {s = \\_ => s} ; 
 
 ----2 Adverbs
   mkAdv : Str -> Adv = \str -> {s =  str ; lock_Adv = <>};

src/persian/StructuralPes.gf

@@ -49,7 +49,7 @@ concrete StructuralPes of Structural = CatPes **
   more_CAdv = {s = "بیشتر" ; p = "" } ; 
   most_Predet = ss "اکثر";
   much_Det = mkDet ["مقدار زیادی"]  Pl ;
-  must_VV = mkV_1 " بایستن " ** {isAux = True} ;  ---- AR
+  must_VV = invarV " بایستن " ** {isAux = True} ;  ---- AR
 --  must_VV = {
 --    s = table {
 --      VVF VInf => ["هوe تْ"] ;