 Definition 2.3 (Complete partial orders). A complete partial order (cpo) is a partially ordered set \({\langle D,\sqsubseteq \rangle}\) with a least element \(\bot\), and such that every increasing chain \(x_0 \sqsubseteq x_1 \sqsubseteq \ldots\) of elements of \(D\) has a least upper bound \(\bigsqcup_n x_n\).