如何找到给定歧义语法中目标句子的第二个最左推导？

嘿，我来帮你理清这个困惑！你已经找到了第一种最左推导，但其实还有另一条完全不同的推导路径——正是这两种路径让句子产生了歧义：到底是女孩碰了拿着花的男孩，还是女孩用花碰了男孩。

首先先明确给定的语法规则：

⟨SENTENCE⟩→⟨NOUN-PHRASE⟩⟨VERB-PHRASE⟩
⟨NOUN-PHRASE⟩→⟨CMPLX-NOUN⟩ | ⟨CMPLX-NOUN⟩⟨PREP-PHRASE⟩
⟨VERB-PHRASE⟩→⟨CMPLX-VERB⟩ | ⟨CMPLX-VERB⟩⟨PREP-PHRASE⟩
⟨PREP-PHRASE⟩→⟨PREP⟩⟨CMPLX-NOUN⟩
⟨CMPLX-NOUN⟩→⟨ARTICLE⟩⟨NOUN⟩
⟨CMPLX-VERB⟩→⟨VERB⟩ | ⟨VERB⟩⟨NOUN-PHRASE⟩
⟨ARTICLE⟩ → a | the
⟨NOUN⟩ → boy | girl | flower
⟨VERB⟩ → touches | likes | sees
⟨PREP⟩ → with

第一种最左推导（语义：女孩碰了拿着花的男孩） #

这是你已经找到的推导路径，核心是把with the flower作为修饰the boy的介词短语：

• ⟨SENTENCE⟩ → ⟨NOUN-PHRASE⟩⟨VERB-PHRASE⟩
• ⟨NOUN-PHRASE⟩ → ⟨CMPLX-NOUN⟩ ⟨VERB-PHRASE⟩
• ⟨CMPLX-NOUN⟩ → ⟨ARTICLE⟩⟨NOUN⟩ ⟨VERB-PHRASE⟩
• ⟨ARTICLE⟩ → the ⟨NOUN⟩ ⟨VERB-PHRASE⟩
• ⟨NOUN⟩ → the girl ⟨VERB-PHRASE⟩
• ⟨VERB-PHRASE⟩ → the girl ⟨CMPLX-VERB⟩
• ⟨CMPLX-VERB⟩ → the girl ⟨VERB⟩⟨NOUN-PHRASE⟩
• ⟨VERB⟩ → the girl touches ⟨NOUN-PHRASE⟩
• ⟨NOUN-PHRASE⟩ → the girl touches ⟨CMPLX-NOUN⟩⟨PREP-PHRASE⟩
• ⟨CMPLX-NOUN⟩ → the girl touches ⟨ARTICLE⟩⟨NOUN⟩⟨PREP-PHRASE⟩
• ⟨ARTICLE⟩ → the girl touches the ⟨NOUN⟩⟨PREP-PHRASE⟩
• ⟨NOUN⟩ → the girl touches the boy ⟨PREP-PHRASE⟩
• ⟨PREP-PHRASE⟩ → the girl touches the boy ⟨PREP⟩⟨CMPLX-NOUN⟩
• ⟨PREP⟩ → the girl touches the boy with ⟨CMPLX-NOUN⟩
• ⟨CMPLX-NOUN⟩ → the girl touches the boy with ⟨ARTICLE⟩⟨NOUN⟩
• ⟨ARTICLE⟩ → the girl touches the boy with the ⟨NOUN⟩
• ⟨NOUN⟩ → the girl touches the boy with the flower

第二种最左推导（语义：女孩用花碰了男孩） #

这条路径的核心是把with the flower作为修饰整个动词短语touches the boy的介词短语，关键区别在对⟨VERB-PHRASE⟩的选择：

• ⟨SENTENCE⟩ → ⟨NOUN-PHRASE⟩⟨VERB-PHRASE⟩
• ⟨NOUN-PHRASE⟩ → ⟨CMPLX-NOUN⟩ ⟨VERB-PHRASE⟩
• ⟨CMPLX-NOUN⟩ → ⟨ARTICLE⟩⟨NOUN⟩ ⟨VERB-PHRASE⟩
• ⟨ARTICLE⟩ → the ⟨NOUN⟩ ⟨VERB-PHRASE⟩
• ⟨NOUN⟩ → the girl ⟨VERB-PHRASE⟩
• ⟨VERB-PHRASE⟩ → the girl ⟨CMPLX-VERB⟩⟨PREP-PHRASE⟩
• ⟨CMPLX-VERB⟩ → the girl ⟨VERB⟩⟨NOUN-PHRASE⟩ ⟨PREP-PHRASE⟩
• ⟨VERB⟩ → the girl touches ⟨NOUN-PHRASE⟩ ⟨PREP-PHRASE⟩
• ⟨NOUN-PHRASE⟩ → the girl touches ⟨CMPLX-NOUN⟩ ⟨PREP-PHRASE⟩
• ⟨CMPLX-NOUN⟩ → the girl touches ⟨ARTICLE⟩⟨NOUN⟩ ⟨PREP-PHRASE⟩
• ⟨ARTICLE⟩ → the girl touches the ⟨NOUN⟩ ⟨PREP-PHRASE⟩
• ⟨NOUN⟩ → the girl touches the boy ⟨PREP-PHRASE⟩
• ⟨PREP-PHRASE⟩ → the girl touches the boy ⟨PREP⟩⟨CMPLX-NOUN⟩
• ⟨PREP⟩ → the girl touches the boy with ⟨CMPLX-NOUN⟩
• ⟨CMPLX-NOUN⟩ → the girl touches the boy with ⟨ARTICLE⟩⟨NOUN⟩
• ⟨ARTICLE⟩ → the girl touches the boy with the ⟨NOUN⟩
• ⟨NOUN⟩ → the girl touches the boy with the flower

你看，两种推导最终得到了同一个句子，但中间步骤的非终结符替换选择不同——第一种是给⟨NOUN-PHRASE⟩加了介词短语，第二种是给⟨VERB-PHRASE⟩加了介词短语，这就导致了语义上的歧义，也证明了这个语法是有歧义的。

内容的提问来源于stack exchange，提问作者Matthew Wyatt