On Tue, Jul 18, 2017 at 3:29 AM, Marc Nieper-Wißkirchen <xxxxxx@nieper-wisskirchen.de> wrote:

"It is an error for any procedure defined in this SRFI to be invoked on mappings with distinct comparators (in the sense of eq?)."

were amended by

"However, implementations are free to extend these procedures so that they may also be invoked on mappings with different comparators but with the same type predicate, whenever it makes sense."

There is no need to add such language; it is implicit in the Scheme use of "it is an error".  It is an error for `car` to be applied to anything but a pair, which means the user can't rely on what will happen; but it is perfectly legal for an implementation to extend the meaning of `car` to an ipair, for example.