Actually, not mine but standard -- just not in Sonja or Pije, as far as I can recall. It follows from the general characterization of a 'la' phrase as setting the context within which the following sentence is true.
tp is mainly a written language and so colloquialisms are rare, except in a few texts with dialog. In those texts, fragments are almost as common as they are in any spoken language, somewhere around 50%. A linear grammar has to just allow that any well-formed sentence part can turn up as an utterance, a de facto sentence, and count as grammatical. A deeper grammar might reject some as not having recoverable antecedents, but such thoughts are not available in your sort of grammar.