Definition 2. 22: The elements a and b of a poset(A, < )are called comparable if either a≤borb≤a. When a and b are elements of A such that neither ab nor b<a, a and b are called incomparableDefinition 2.22：The elements a and b of a poset (A, ≼) are called comparable if either a≼b or b≼a. When a and b are elements of A such that neither a≼b nor b≼a, a and b are called incomparable