Scenario A: A text-infilling model is tasked with completing the sentence 'The dog ___ in the park.' It replaces the blank, which could represent any number of words (e.g., 'played', 'was chasing a ball'), with a single special symbol. The model must then generate an appropriate sequence of words to replace that one symbol.

Scenario B: An early machine translation system is translating a French sentence containing the word 'au' into English. Depending on the context, this single French word might need to be translated into two English words, 'to the'.

Explain the fundamental modeling challenge that is common to both of these scenarios.