So if some client code, say code that results from the expansion of
  the DEFMETHOD macro would like to use ENSURE-GENERIC-FUNCTION to
  make sure a generic function with the appropriate name exists, it
  can not do so, because it would run the risk or supplying the wrong
  values of one or more of these keyword arguments, thereby
  inadvertently modifying an existing generic function.
  
-> (commas moved around, and (maybe?) arguments -> parameters)

  So if some client code, say code that results from the expansion of
  the DEFMETHOD macro, would like to use ENSURE-GENERIC-FUNCTION to
  make sure a generic function with the appropriate name exists, it
  cannot do so because it would run the risk or supplying the wrong
  values of one or more of these keyword parameters, thereby
  inadvertently modifying an existing generic function.