 A tree \(\mathcal{T} = \left\langle T, \prec \right\rangle\) depicts alternative, mutually incompatible possibilities for the future in a unified structure, and it can be carved up into multiple histories. A history in \(\mathcal{T}\) is a maximal (i.e. non-extendable) set of time instants that is linearly ordered by \(\prec\), that is, an entire path through the tree; and it represents a complete possible course of events. We denote the set of all histories in \(\mathcal{T}\) by \(\mathcal{H}(\mathcal{T})\), and we use \(\mathcal{H}(t)\) for the set of histories passing through a given time instant \(t\) in \(\mathcal{T}\). 