(NC-140) Groups can have only one rule.

Rule conditions can be arbitrary boolean expressions anyways, so this
doesn't remove any functionality, it just simplifies classification
since the user doesn't have to wonder whether multiple rules are ANDed
or ORred together; instead they just explicitly make the rule
a conjuction or disjunction as desired.
(NC-140) Update group rule using delta.

The semantics of updating the rule are the same as updating the
environment: if the key is in the delta, its value replaces the old
wholesale; otherwise, the rule is left alone.