thm". It conducts a depth-first search
of the pattern tree. That is, it proceeds along a single path through the tree,
checking that the subject matches what is required. When there is a mismatch,
the algorithm tries any alternatives at the current point, and if they all
fail, it backs up to the previous branch point in the tree, and tries the next
alternative branch at that level. This often involves backing up (moving to the
left) in the subject string as well. The order in which repetition branches are
t