Add one value to the indicated pattern attribute. The names of attributes
are the same as the Tooltalk accessors used to set them less the
"tooltalk_pattern_" prefix and the "_add" suffix). For example
-the name of the attribute for tt_pattern_dispostion_add attribute
+the name of the attribute for tt_pattern_disposition_add attribute
is 'disposition. The 'category attribute is handled specially,
since a pattern can only be a member of one category (TT_OBSERVE
or TT_HANDLE.