webservice endpoints should expose which groups are currently assigned to a grouping

Currently, none of the web-service endpoints under core_groups  appears to provide insights on which groups belong to a grouping. Maybe I am missing something, but I find it impossible to link group and grouping infos for a course through the web-service API.

It would be nice to be able to verify if a group is assigned to a grouping from a web-service API endpoint. 

I would consider this a bug in the API, but since the endpoints are around for a while, it may well be a simple improvement. 

I see  two potential options to solve this: 

1. Add a property to the group objects resulting from core_group_get_course_groups for a grouping-id, if a group is assigned to a grouping. 
2. Add a list property to a grouping object resulting from core_group_get_course_groupings, which contains the related group ids (or even entire group objects). 

Personally, I would prefer option 2 if only one endpoint could change. 

Both solutions should not affect any existing API implementations  as no existing properties are changed. 

I was unsuccessful using calls to the following endpoints for retrieving links between groups and groupings:

• core_group_get_groups
• core_group_get_groupings
• core_group_get_course_groups
• core_group_get_course_groupings
• core_group_get_course_user_groups
• core_group_get_group_members