Graql: sub-relationships should be able to declare new roles that does not override one of the roles in the super-relationships

[grabl]

This issue was originally posted by @haikalpribadi on 2018-03-27 14:45.

For example:

general-data-link sub relationship, relates linked-data,

special-data-link sub general-data-link,   relates special-linked-data as linked-data, # overrides super role   relates some-reference, # new role has special-name;