Halliday & Matthiessen (1999: 288-9):
It is the fact that metaphor multiplies meanings within the semantic system that opens up the possibility of metaphorical chains, with one congruent starting–point and another highly metaphorical end-point (A’’’ stands for A’’ stands for A’ stands for A … The semantic system is being expanded along the dimension of the metaphorical token–value relation; but the expansion is still within the semantic system itself.