class-to-only-child := (: BTreeEqEOF BTreeEq<String,String>);

has-only-child := λ(: class String)(: tag String). (: (
   (.bind( (as (& class-to-only-child) BTreeEq<String,String>[]) class tag ))
) Nil);

get-only-child := λ(: class String). (: (
   (let r (.lookup( class-to-only-child class '_s )))
   r
) String);