
  [;1m-spec family_to_relation(Family) -> BinRel[0m
  [;1m                            when[0m
  [;1m                                Family :: family(),[0m
  [;1m                                BinRel :: binary_relation().[0m

  If [;;4mFamily[0m is a family, then [;;4mBinRel[0m is the binary relation
  containing all pairs (i, x) such that i belongs to the index set
  of [;;4mFamily[0m and x belongs to [;;4mFamily[0m[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}]
