Grammar re;
Lexer
        //alt1,2,3 a = 'a'; b = 'a' | 'b' ;
        //alt4,5 a = 'a' | 'c' ; b = 'a' | 'b' ;
        //alt6,7 a = 'a' ; b = 'b' ;
        //alt8 a = ('a' Lookahead 'b') | 'c'; b = 'a';
        //alt9,10 a = 'a'; b = 'b';

xa = a; xb =b; //alt2,5,7,10 xa = a ; xb = b - a; //Precedence a > b; //alt3 xa = a - b ; xb = b; //Precedence b > a;
blank = #10 | #13;

Parser
Ignored blank;
t = xa | xb;