A mixin class to get methods that provide easy access to member
variables in the %$self.
Ideally there should be better Perl language support for this.
There is only one method provided:
_elem($elem [, $val])
Internal method to get/set the value of member variable
$elem. If $val is present it is used as the new value
for the member variable. If it is not present the current
value is not touched. In both cases the previous value of
the member variable is returned.
