Lemke (1988: 159) reinterpreted his 'covariate structure' as a structuring principle, rather than a kind of structure:
My own 'covariate structure' (Lemke 1985), which includes Halliday's univariate type, is for the case of homogeneous relations of co-classed units, and should perhaps be called a 'structuring principle' rather than a kind of structure.
