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

index-fields-of-tag := λ(: tag String)(: fields-params Type)(: fields-rhs Type). (: (
   (.bind( (as (& tag-to-fields-index) BTreeEq<String,Tuple<Type,Type>>[]) tag (Tuple( fields-params fields-rhs )) ))
) Nil);

fields-of-tag := λ(: tag String). (: (
   (let r (.lookup( tag-to-fields-index tag (Tuple( TAny TAny )) )))
   r
) Tuple<Type,Type>);

fields-of-tag := λ(: tag Type)(: class Type). (: (
   (let rhst TAny)
   (match tag (
      ()
      ( (TGround( 'Tag_s (LCons( (TGround( ts _ )) LEOF )) )) (match (fields-of-tag ts) (
         ()
         ( (Tuple( TAny TAny )) () )
         ( (Tuple( fields-lhs fields-rhs )) (
            (set rhst (substitute( (unify( fields-lhs class )) fields-rhs )))
         ))
      )))
      ( _ (
         (print 'Invalid\sTag\sIn\sfields-of-tag\s_s)
         (print tag)(print '\n_s)
         (exit 1_u64)
      ))
   ))
   (set rhst (with-size rhst))
   (let rt (TGround( 'Fields_s (close(cons(
      (fields-of-type( (t3( 'Cons_s rhst (TAnd( (close(t1 'U64_s)) (close(t2( 'Sized_s (t1 '8_s) ))) )) )) ))
      (: LEOF List<Type>)
   ))))))
   rt
) Type);