An unconditional 'X' rule has already been entered, so no other 'X'
rules are permitted.
or Previous 'X' rules have conditions, so the current one requires one as well.
Compilation continues but no compiled source code file will be saved.
Rules such as #IMPLIED element rules, marked-section rules, markup-error
rules, etc., can be entered either once with no attached condition, or
several times as long as each rule header has a condition.