
  -spec family_to_relation(Family) -> BinRel
                              when
                                  Family :: family(),
                                  BinRel :: binary_relation().

  If Family is a family, then BinRel is the binary relation
  containing all pairs (i, x) such that i belongs to the index set
  of Family and x belongs to Family[i].

    1> F = sofs:family([{a,[]}, {b,[1]}, {c,[2,3]}]),
    R = sofs:family_to_relation(F),
    sofs:to_external(R).
    [{b,1},{c,2},{c,3}]
