<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def set_default(self, val, force=False):
 """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. """

        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"
            )