  Abductive forgetting is removing variables from a logical formula while
maintaining its abductive explanations. It is defined in either of two ways,
depending on its intended application. Both differ from the usual forgetting,
which maintains consequences rather than explanations. Differently from that,
abductive forgetting from a propositional formula may not be expressed by any
propositional formula. A necessary and sufficient condition tells when it is.
Checking this condition is \P{3}-complete, and is in \P{4} if minimality of
explanations is required. A way to guarantee expressibility of abductive
forgetting is to switch from propositional to default logic. Another is to
introduce new variables.
