1. A method of verifying a simulated system's compliance with a defined system specification, wherein the system specification has explicitly defined don't care conditions, the method comprising: for each incompletely specified function represented by an ordered pair (f,d) wherein f is an on-set and wherein d is a don't care set, transforming the ordered pair representation into an interval represented by an ordered pair [m, M] wherein [m, M]={f, m .OR right.f.OR right.M}; propagating a set of input conditions through such interval to produce a set of output conditions representative of such on-set and of such don't care set; and verifying that the set of output conditions meets the defined system specification.