Halliday & Matthiessen (2014: 450):
… internal nesting always occurs when there is a change in taxis. That is, any logical sequence of clauses is always either paratactic (1 2 3 …) or hypotactic (α β γ …). It is never a mixture of the two; … If there is a switch in taxis, this automatically leads to nesting … . By the same token, any logical sequence of clauses is always constant in logico-semantic type — projection of ideas, projection of locutions, elaboration, extension or enhancement. It is never a mixture of types; … If there is a switch in logico-semantic type, then nesting automatically occurs …