class_docstr: This class provides a set of methods to read, parse and answer
  questions about a single robots.txt file.
code: def request_rate(self, useragent):
      if not self.mtime():
          return None
      for entry in self.entries:
          if entry.applies_to(useragent):
              return entry.req_rate
      if self.default_entry:
          return self.default_entry.req_rate
      return None
  
imports: import collections
  import urllib.parse
  import urllib.request
label_desc: Use image node_id 10 for calling the RobotFileParser obj's underlying member method code with example usage: obj.request_rate(useragent) and returns: None, None, self, entry