(* Env: environments that map keys of type 'key to data of type 'data *)

type ('key, 'data) env

val empty    : (''key, 'data) env
val lookup   : (''key, 'data) env -> ''key -> 'data 
val bind1    : (''key, 'data) env -> ''key * 'data -> (''key, 'data) env
val bind     : (''key, 'data) env -> (''key * 'data) list -> (''key, 'data) env
val bindZip  : (''key, 'data) env -> ''key list * 'data list 
               -> (''key, 'data) env
val fromList : (''key * 'data) list -> (''key, 'data) env
val plus     : (''key, 'data) env * (''key, 'data) env -> (''key, 'data) env