Allow exceptions to a set of rules rather than one rule

[denismerigoux]

Allows an exception to be specified to a group of rules rather than just one rule. This is done by grouping together rules with the same label:

scope Foo:
  label l definition x under condition y < 0 consequence equals true
  label l definition x under condition y >= 0 consequence equals false
  exception l definition x under condition y = 0 consequence equals true