lone-index := (: BTreeEqEOF BTreeEq<String,U64>);

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

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