An unconditional rule for the external entity 'X' has
already been entered, so no other rules are permitted.
or Previous rules for the external entity 'X' have conditions, so the
current one requires one as well.
Compilation continues but no compiled source code file will be saved.
It is an error if more than one external-entity rule can be
selected at any one time and any particular external entity can appear
in either only one rule without a condition or several conditional