@uml All tokens go to the parser (unless skip() is called in that rule) on a particular "channel". The parser tunes to a particular channel so that whitespace etc... can go to the parser on a "hidden" channel.
@uml During lookahead operations, this "token" signifies we hit rule end ATN state and did not follow it despite needing to.
@uml Anything on different channel than DEFAULT_CHANNEL is not parsed by parser.
@uml This is the minimum constant value which can be assigned to a ser-defined token channel.
@uml Token constant definitions