Pruning based on word pairs is performed in the initial stage of parsing, before any structure is built above the words. Some parts of the sentence will be excised and made into modifiers after building of structure has commenced - negation, adverbs, some prepositional chains, parenthetical phrases, etc., - causing new word pairs to appear after removal of the modifiers. The ANDWITH operator is necessary, both to prevent errors being made due to insufficient pruning in the initial stage, and to re-initiate pruning at the word-pair level after excision, and while some structure may already exist above the words.
a share buy back
The "buy" can be a noun or a verb. But "share" is a noun (forced by the article), and it is singular. The present tense third person singular form of "buy" would be "buys". If a word can be either a noun or a verb, we can prune the verb possibility because the number won't match. The ANDWITH operator will only allow through to the ALTERNATIVES operator a word that has both options.
prevents the alternative of NounPluralVerb being reached unless both options are present. A NOTCONSISTENT operator used as
will then cut the verb option.