 According to Veltman (2005), when you are “retracting” a fact from the facts in the context, you begin by considering each \(w\in \textit{Facts}_C\) and find the smallest situations in w which contain only undetermined facts—he calls such a situation a basis for w. This is a minimal situation which, given the laws constraining \(\textit{Universe}_C\), determines all the other facts about that world. For example, \(w_6\) has only one basis, namely \(s_0={\{{\langle \mathsf{R},1\rangle},{\langle \mathsf{H},0\rangle}\}}\), and \(w_7\) has only one basis, namely \(s_1={\{{\langle \mathsf{R},1\rangle},{\langle \mathsf{H},1\rangle}\}}\). Once you have the bases for a world, you can retract a fact by finding the smallest change to the basis that no longer forces that fact to be true. So retracting the fact that \(\mathsf{R}\) is true from \(s_0\) produces \(s'_0={\{{\langle \mathsf{H},0\rangle}\}}\), and retracting it from \(s_1\) produces \(s'_1={\{{\langle \mathsf{H},1\rangle}\}}\). The set consisting of these two situations is the premise set.