tag-to-class-index := (: BTreeEqEOF BTreeEq<String,Type>);

index-class-of-tag := λ(: tag String)(: class Type). (: (
   (.bind( (as (& tag-to-class-index) BTreeEq<String,Type>[]) tag class ))
   (match class (
      ()
      ( (TGround( cls _ )) (
         (.bind( (as (& class-to-yes-index) BTreeEq<String,U64>[]) cls 1_u64 ))
      ))
      ( _ (
         (print 'Unexpected\sClass\sIn\sindex-class-of-tag:\s_s)(print class)(print '\n_s)
         (exit 1_u64)
      ))
   ))
) Nil);

class-of-tag := λ(: tag String). (: (
   (let r (.lookup( tag-to-class-index tag TAny )))
   r
) Type);