In greedy regexps first alternative must correspond to consuming path.

In greedy regexps first alternative must correspond to consuming path.

By convention first alternative has higher priority.
So, for example, the following must be true:
    r*     = rr* | <empty>
    r{n,m} = r{n} (r{m - n} | r{m - n - 1} | ... | r{1} | <empty>)
    r{n,}  = r{n} (rr* | <empty>)

For now we don't care about priorities: this is a preparatory step
before transition to greedy leftmost semantics for tags.