yams #242339 [schema] provide a .relation(role) method [in-progress]

Will help avoid the nasty pattern:

for role in ('subject', 'object'):
    for rschema in getattr(eschema, '%s_relations' % role)():
        pass
prioritynormal
typeenhancement
done in<not specified>
closed by<not specified>
patch[schema] add a .relations(role) method [in-progress]