Define an external data entity named ename with type typ (CDATA, NDATA or SDATA) and notation not.
This command will be preceded by one or more f-commands giving the filenames generated by the entity manager from the system and public identifiers, by a p-command if a public identifier was declared for the entity, and by a s-command if a system identifier was declared for the entity, not will have been defined using a N-command.
Data attributes may be specified for the entity using D-commands.
An external data entity will only be defined if it is to be referenced in a &-command or in an A-command for an attribute whose declared value is ENTITY or ENTITIES.
