Improper­Circular­Lists

Here are the valid uses of improper lists in R7RS-small in contexts where lists (as opposed to arbitrary objects) are expected:

• As a lambda-list
• As the last argument of append
• As the argument to list-copy, which then returns an improper list that is equal? to the argument

And of circular lists:

• As the list argument to list-ref
• As the list arguments to map and for-each (though it is an error if all arguments are circular)