def flat_map(include_self = false, &block)
      result = []

      each_expression(include_self) do |exp, index|
        if block_given?
          result << yield(exp, index)
        else
          result << [exp, index]
        end
      end

      result
    end
def flat_map(include_self = false, &block)
      result = []

      each_expression(include_self) do |exp, index|
        if block_given?
          result << yield(exp, index)
        else
          result << [exp, index]
        end
      end

      result
    end
Returns a new array with the results of calling the given block once
 for every expression. If a block is not given, returns an array with
 each expression and its level index as an array.