# File lib/core/facets/enumerable/one.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