Create an entity file that defines general entities to include each chapter and guard these definitions with a parameter entity that can be set to either
<literal>INCLUDE
</literal> or
<literal>IGNORE
</literal> to control whether the entity is defined. After these conditional general entity definitions, place one more definition for each general entity to set them to an empty value. This technique makes use of the fact that entity definitions cannot be overridden but the first definition always takes effect. So the inclusion of the chapter is controlled with the corresponding parameter entity. Set to
<literal>INCLUDE
</literal>, the first general entity definition will be read and the second one will be ignored. Set to
<literal>IGNORE
</literal>, the first definition will be ignored and the second one will take effect.