
  -spec intersection_of_family(Family) -> Set
                                  when Family :: family(), Set :: a_set().

  Returns the intersection of family Family.

  Intersecting an empty family exits the process with a badarg
  message.

    1> F = sofs:family([{a,[0,2,4]},{b,[0,1,2]},{c,[2,3]}]),
    S = sofs:intersection_of_family(F),
    sofs:to_external(S).
    [2]
