code: def _process_plan_lines(self, final_line_count):
          
          if not self._lines_seen["plan"]:
              self._add_error(_("Missing a plan."))
              return
  
          if len(self._lines_seen["plan"]) > 1:
              self._add_error(_("Only one plan line is permitted per file."))
              return
  
          plan, at_line = self._lines_seen["plan"][0]
          if not self._plan_on_valid_line(at_line, final_line_count):
              self._add_error(
                  _("A plan must appear at the beginning or end of the file.")
              )
              return
  
          if plan.expected_tests != self._lines_seen["test"]:
              self._add_error(
                  _("Expected {expected_count} tests but only {seen_count} ran.").format(
                      expected_count=plan.expected_tests,
                      seen_count=self._lines_seen["test"],
                  )
              )
docs: Process plan line rules.