Get rid of (mandatory) attributes

[h908714124]

Attributes can always be inferred:

1. collector absent, mapper returns optional type -> optional
2. collector absent, mapper returns list -> repeatable
3. collector absent, mapper returns anything else but boolean -> not optional, not repeatable (~required)
4. collector present -> repeatable, not optional; explanation:
   1. collector doesn't return optional type -> repeatable, not optional
   2. collector returns optional -> repeatable (otherwise collector won't be used), not optional (otherwise collector won't be used)