helps define when exactly sponsorship can occur, and so while not technically necessary, it is useful for the purpose of brevity.

operations occur between arcs when it becomes necessary to specify which linguistic level is phonologically attested. Where two arcs share the same grammatical relation to the same root node, the one at the surface level erases the one at lower levels. Successors always erase their predecessors, except in one case.

Limitations

There are very few

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