  This paper uses possible-world semantics to model the changes that may occur
in an agent's knowledge as she loses information. This builds on previous work
in which the agent may forget the truth-value of an atomic proposition, to a
more general case where she may forget the truth-value of a propositional
formula. The generalization poses some challenges, since in order to forget
whether a complex proposition $\pi$ is the case, the agent must also lose
information about the propositional atoms that appear in it, and there is no
unambiguous way to go about this.
  We resolve this situation by considering expressions of the form
$[\boldsymbol{\ddagger} \pi]\varphi$, which quantify over all possible (but
minimal) ways of forgetting whether $\pi$. Propositional atoms are modified
non-deterministically, although uniformly, in all possible worlds. We then
represent this within action model logic in order to give a sound and complete
axiomatization for a logic with knowledge and forgetting. Finally, some
variants are discussed, such as when an agent forgets $\pi$ (rather than
forgets whether $\pi$) and when the modification of atomic facts is done
non-uniformly throughout the model.
