# File lib/active_support/caching_tools.rb, line 31
      def hash_cache(method_name, options = {})
        selector = options[:as] || "#{method_name}_cache"
        method = self.instance_method(method_name)
        
        args = []
        code = "def #{selector}(); @#{selector} ||= "
        
        (1..method.arity).each do |n|
          args << "v#{n}"
          code << "Hash.new {|h#{n}, v#{n}| h#{n}[v#{n}] = "
        end
        
        # Add the method call with arguments, followed by closing braces and end.
        code << "#{method_name}(#{args * ', '}) #{'}' * method.arity} end"
        
        # Extract the line number information from the caller. Exceptions arising
        # in the generated code should point to the +hash_cache :...+ line.
        if caller[0] && /^(.*):(\d+)$/ =~ caller[0]
          file, line_number = $1, $2.to_i
        else # We can't give good trackback info; fallback to this line:
          file, line_number = __FILE__, __LINE__
        end
        
        # We use eval rather than building proc's because it allows us to avoid
        # linking the Hash's to this method's binding. Experience has shown that
        # doing so can cause obtuse memory leaks.
        class_eval code, file, line_number
      end