This class defines the following public API
-=head2 set_attribute ( $attr=>value | {name=>$attr,value=>$value} )
+=head2 set_attribute
Sets an attribute of a given name to a given value for all matching selections.
calls are made against the same or overlapping selection sets, the final
call wins.
-=head2 add_to_attribute ( $attr=>value | {name=>$attr,value=>$value} )
+=head2 add_to_attribute
Adds a value to an existing attribute, or creates one if the attribute does not
yet exist.
Attributes with more than one value will have a dividing space.
-=head2 remove_attribute ( $attr | {name=>$attr} )
+=head2 remove_attribute
Removes an attribute and all its values.