7.1.1 Syntax (Revised Report, p 99)

m) WHETHER MOID1 is firm MOID2:

WHETHER MOID1 equivalent MOID2

or MOID1 unites to MOID2

or MOID1 deprefs to firm MOID2.

The possitility or otherwise of coercisons between two modes is determined by the predicate 'is firm'.