Halliday & Matthiessen (1999: 34):
As we have already noted, the representational semiotic may be language itself — the theory may be represented discursively in the register(s) of linguistics. In this case, the relationship between theory and language is […] a relationship where theory might be construed as a connotative semiotic (in Hjelmslev’s, 1943, conception: a semiotic system whose expression plane is a semiotic system) …