Still there are even more complex situations. One of them is the system \(\bR\) of relevance logic. Consider the class of algebras \(\bRal\) defined in Font &amp; Rodríguez 1990 (see also Font &amp; Rodríguez 1994) and denoted there by ‘\(\bR\)’. Let us consider for every \(\bA \in \bRal\) the set \[\tE(\bA) := \{a \in A: a \wedge^{\bA }(a \rightarrow^{\bA } a) = a \rightarrow^{\bA } a\}.\] Then \(\bRal\) is said to be an algebraic semantics for \(\bR\) because the following holds: