Confusing Formal Constituency (Rank) With Function Structure

Martin (2013: 62):

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To be clear, functions are not constituents of rank units, such as clause and nominal groups. Constituents of rank units are lower rank units on the rank scale. Constituents of clauses are groups and phrases.

The Thing is a function of a word, a constituent of a nominal group, and the structure of a rank unit is the relationships among the functions, not the functions themselves (Halliday & Matthiessen 2014: 451).