Since LEFT-ARC(N) and RIGHT-ARC(N) result in the removal of a daughter item from the stack or queue, which may be required in subsequent parsing stages, it is vital to ensure that the daughter has collected all and only its daughters.
subtrees can be used to encourage the parser to remove a daughter item only if there is evidence that it has collected all and only its daughters, this corresponds to completeness and cohesion in Lexical Functional Grammar (LFG) (Bresnan and Kaplan, 1982).