A propositional directed acyclic graph (PDAG) is a data structure that is used to represent a Boolean function. A Boolean function can be represented as a rooted, directed acyclic graph of the following form: * Leaves are labeled with (true), (false), or a Boolean variable. * Non-leaves are (logical and), (logical or) and (logical not). * - and -nodes have at least one child. * -nodes have exactly one child.