{ $class-description "The class of bidirectional assocs. Bidirectional assoc are implemented by combining two assocs, with one the transpose of the other." } ;
"A " { $emphasis "bidirectional assoc" } " combines a pair of assocs to form a data structure where both normal assoc opeartions (eg, " { $link at } "), as well as " { $link "assocs-values" } " (eg, " { $link value-at } ") run in sub-linear time."
"Bidirectional assocs implement the entire " { $link "assocs-protocol" } " with the exception of " { $link delete-at } ". Duplicate values are allowed, however value lookups with " { $link value-at } " only return the first key that a given value was stored with."