tag-to-index-index := (: BTreeEqEOF BTreeEq<String,U64>);
unknown-index-of-tag := 99999_u64;

index-of-tag := λ(: tag String). (: (
   (let r (.lookup( tag-to-index-index tag unknown-index-of-tag )))
   r
) U64);

index-index-of-tag := λ(: tag String)(: index U64). (: (
   (.bind( (as (& tag-to-index-index) BTreeEq<String,U64>[]) tag index ))
) Nil);