
  -spec relation(Tuples) -> Relation
                    when Relation :: relation(), Tuples :: [tuple()].

  Creates a relation. relation(R, T) is equivalent to 
  from_term(R, T), if T is a type and the result is a relation.
  If Type is an integer N, then [{atom, ..., atom}]), where the
  tuple size is N, is used as type of the relation. If no type is
  explicitly specified, the size of the first tuple of Tuples is
  used if there is such a tuple. relation([]) is equivalent to 
  relation([], 2).
