[plt-scheme] Structures added to MzLib

> The (open path) form may appear anywhere a (define ...) form 
> can appear. The open form places all of the identifiers 
> exported from the structure indicated by the path into the 
> context of the open expression (the binding context of the 
> first identifier in the path is used).

Are there any checks for duplicate bindings when open'ing a structure?

Ocaml allows an open to shadow a previous binding:

	# open List;;
	# map;;
	- : ('a -> 'b) -> 'a list -> 'b list = <fun>
	# open Array;; 
	# map;;
	- : ('a -> 'b) -> 'a array -> 'b array = <fun>

-- Paul