- --check is slightly f***ed up because there is no good semantics for
  the --disable intruder check. As a result, it is now too strict can
  cause correct protocols to fail. Fix.
- When *not* asking for attack output, maybe we should default to
  --prune = 1. Then, if we ask for --xml output or --dot, we do:
  if --prune == 1 then --prune == 2 now :) unless otherwise specified.
  (This should be done after switch checking)
- Old version enforced some extra orders:
  1. M_0 roles were ordered before any other roles.
  2. Local constants order: if a run has a local variable instantiated by
  somebody else's variable, that should occur then after the initial sending
  of that value...
- Test 'sk(x)' in goals, somewhere before assessing a state (dus at the
  beginning of iterate), immediately reduce to 'sk(Eve)'. Test with
  --experimental. To that end, reintroduce a state-reporting switch.
- It is currently not well-defined to define inversekeys within a role:
  this requires some work at instantiation, because instantiated term
  couples should be added to the inverses list, and removed at
  descruction.
- Simple timestamps could be added by prefixing send message before the
  role, sending any timestamp constants out first to the intruder. These
  should of course be hidden in the output somehow.
- Notes on the new attack group displays:
  * We want to group runs into consistent protocol runs.
  * Minimal req. for protocol run: equal \rho.
  * If two runs are candidates for a role in a protocol run,
    use a metric based on order and data. Maybe data is more important:
    if equal data, than order might be irrelevant.
  * Maybe we should refactor the xmlOut code first. In an extreme case,
    we first factor out all logic, and ranking, and grouping, in to a
    prepareAttackOutput structure; with a separate source file. Later we
    can convert this to either ASCII or DOT or XML or something.
    Now that I think of it; XML should be a plain state probably, and we
    could add a switch to also output more detailed attack things (is
    that relevant?)
- SConstruct file should check whether ctags actually exists (avoiding
  errors)
- Proof output should be XML, with an external converter to dot format.