code: def set_default(self, val, force=False):
          
          if self.default is None or force:
              self.default = val
              self.set_value(val)
              self.has_changed = True
          else:
              raise OptionError(
                  "cannot override existing default without using the 'force' "
                  "option"
              )
docs: this function allows a default to be set on an option that dosen't
          have one.  It is used when a base class defines an Option for use in
          derived classes but cannot predict what value would useful to the
          derived classes.  This gives the derived classes the opportunity to
          set a logical default appropriate for the derived class' context.
  
          For example:
  
              class A(RequiredConfig):
                  required_config = Namespace()
                  required_config.add_option(
                    'x',
                    default=None
                  )
  
              class B(A):
                  A.required_config.x.set_default(68)
  
          parameters:
              val - the value for the default
              force - normally this function only works on Options that have not
                      had a default set (default is None).  This boolean allows
                      you to override an existing default.