  As motivation, one might think of \(s\) and \(t\) as two  volumes of an encyclopedia, and \(s\) + \(t\) as the set of those two volumes. Imagine that one of the volumes, say \(s\),  contains a sufficient justification for a proposition \(F\),  i.e., \(s : F\) is the case. Then the larger set  \(s\) + \(t\) also contains a sufficient justification for  \(F\), [\(s\) + \(t ]: F\). In the Logic of  Proofs \(\mathsf{LP}\), Section  1.2, ‘\(s\) + \(t\)’ can be interpreted as a  concatenation of proofs \(s\) and \(t\).