0 = a − a is an element of X and this property does not depend on the choice of a ∈ X. The
ordered set (X;≤) is a semi-Boolean algebra in the sense of [1], that is, it is a meet semilattice
with zero 0 in which every interval [0,a] is a Boolean algebra with respect to induced order.
Here a ∧b = a −(a −b) and the complement of an element b ∈ [0,a] is a −b.