# File lib/facet/enumerable/one%3F.rb, line 21
  def one?  # :yield: e
    matches = 0
    if block_given?
      self.each do |e|
        if yield(e)
          matches += 1
          return false if matches > 1
        end
      end
      return (matches == 1)
    else
      one? { |e| e }
    end
  end