The theme is what is being talked about, the point of departure for the clause as message, and the speaker has within certain limits the option of se… - Michael Halliday

The grammatical system has … a functional input and a structural output; it provides the mechanism for different functions to be combined in one utterance

A physical system is just that: a physical system. What is systematized is matter itself, and the processes in which the system is realized are also material. But a biological system is more complex: it is both biological and physical — it is matter with the added component of life; and a social system is more complex still: it is physical, and biological, with the added component of social order, or value. … A semiotic system is still one step further in complexity: it is physical, and biological, and social —and also semiotic: what is being systematized is meaning. In evolutionary terms, it is a system of the fourth order of complexity

It is part of the task of linguistics to describe texts, and all texts, including those prose or verse, which fall within any definition of literature and are accessible to analysis by the existing methods of linguistics.