[notes]
https://github.com/antlr/antlr4/issues/1070

[type]
Lexer

[grammar]
lexer grammar L;

channels { break }

A: 'a' -> mode(for);

mode for;
B: 'b' -> channel(break);

[input]
ab

[output]
[@0,0:0='a',<1>,1:0]
[@1,1:1='b',<2>,channel=2,1:1]
[@2,2:1='<EOF>',<-1>,1:2]