Lattice ordered group

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It seems the basic fact but I did not get the idea how to prove it. Let $G$ be a partially ordered group. Then $G$ is an $\ell$-group if and only if every pair of positive elements has least upper bound.

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This follows from the assertion:

A partially ordered group $G$ is an $l$-group if and only if for any $a\in G$ there is the least upper bound of $a$ and $1$.

[Fuchs, L. Partially ordered algebraic systems. Pergamon Press, 1963, chapt.5, sec.1]