def each
      return to_enum unless block_given?
      seen = {}
      tuples.each do |tuple|
        tuple = Tuple.coerce(header, tuple)
        yield seen[tuple] = tuple unless seen.key?(tuple)
      end
      self
    end
def each
      return to_enum unless block_given?
      seen = {}
      tuples.each do |tuple|
        tuple = Tuple.coerce(header, tuple)
        yield seen[tuple] = tuple unless seen.key?(tuple)
      end
      self
    end
Iterate over each tuple in the set

 @example
   relation = Relation.new(header, tuples)
   relation.each { |tuple| ... }

 @yield [tuple]

 @yieldparam [Tuple] tuple
   each tuple in the set

 @return [self]

 @api public