def select_first(root)
      stack = [root]
      while node = stack.pop
        if node.tag? && subset = match(node, true)
          return subset.first if !subset.empty?
        elsif children = node.children
          stack.concat children.reverse
        end
      end
      nil
    end
def select_first(root)
      stack = [root]
      while node = stack.pop
        if node.tag? && subset = match(node, true)
          return subset.first if !subset.empty?
        elsif children = node.children
          stack.concat children.reverse
        end
      end
      nil
    end
Similar to #select but returns the first matching element. Returns +nil+
 if no element matches the selector.