Halliday & Matthiessen (1999: 561):
There are of course many different contexts for all these indeterminacies, in different regions of the total semantic space. Certain types of ambiguity appear to be not so much artefacts of the realisation (not just grammatical puns, so to speak) but rather another kind of complementarity, where the grammar is as it were "having things both ways" — both interpretations have to be accepted at one and the same time. This is sometimes the case with Token + Value structures, in figures of being. These clauses are always ambiguous, if the verb is be, since this verb does not mark the passive; yet some depend on being interpreted both ways — particularly, perhaps, some proverbial sayings, Thus, one man's meat is another man's poison is both Token ^ Value 'what one person likes may displease another' and Value ^ Token 'what one person dislikes may please another'; contrast what's sauce for the goose is sauce for the gander, which can be interpreted only as Token ^ Value.