tag-is-fragment-index := (: BTreeEqEOF BTreeEq<String,U64>);

index-tag-is-fragment := λ(: tag String). (: (
   (.bind( (as (& tag-is-fragment-index) BTreeEq<String,U64>[]) tag 1_u64 ))
) Nil);

is-fragment := λ(: tag String). (: (
   (.lookup( tag-is-fragment-index tag 0_u64 ))
) U64);

is-fragment := λ(: tt Type). (: (
   (is-fragment(tag-of tt))
) U64);