def store(actual, truth)
      key = @keys_for.call(truth, actual)

      if @values.key?(key)
        @values[key] += 1
      else
        fail IndexError, "#{key} not found in confusion matrix"
      end

      self
    end
def store(actual, truth)
      key = @keys_for.call(truth, actual)

      if @values.key?(key)
        @values[key] += 1
      else
        fail IndexError, "#{key} not found in confusion matrix"
      end

      self
    end
Save the result of classification

 @param [Object] actual  The classified value
 @param [Object] truth   The known, expected value
 @return [self]