4.2.2.4. Departure from the Unit Rule.Federal courts have repeatedly emphasized that the unit rule is “a ‘carefully guarded’ one and that only in rare and exceptional types of situations [should] departures from it be[ ] permitted.”254 Thus, while the courts recognize the unit rule “manifestly is not without hardships in practical operation,”255 under federal law departure from the unit rule is permitted only in “extraordinary,” “unique,” “rare and compelling” circumstances.256 Any departure from the unit rule requires a legal instruction, as “the determination as to [the unit rule’s] applicability is one made by a court as a matter of law rather than by an appraiser.”257