change (label) into [label]

In the suggested patch for #2, I'm changing the notation from `(label)` to `[label]` to avoid ambiguities with state formulas.

Fanda seems to prefer this notation for labels.  So do I.

I would like to update the label notation now.  This will help improving the patches for #2 (which uses the `[label]` notation) or #5 (which currently uses the `(label)` notation) without creating conflicts.

Do we agree?  (I will update the patches for #2 and #5 after this is merged.)