antlr does not parse when token only mentions one other token - antlr4

Question

I am trying to learn EBNF grammars with ANTLR. So I thought I would convert the Wikipedia EBNF grammar to ANTLR 4 and play with it. However I have had a terrible time at it. I was able to reduce the grammar to the one step that generates the problem.
It seems if I have one token reference solely another token then ANTLR 4 can't parse the input.
Here is my grammar:
grammar Hello;
program : statement+ ;
statement : IDENTIFIER STATEMENTEND /*| LETTERS STATEMENTEND */ ;
LETTERS : [a-z]+ ;
IDENTIFIER : LETTERS ;
SEMICOLON : [;] ;
STATEMENTEND : SEMICOLON NEWLINE* | NEWLINE+ ;
fragment NEWLINE : '\r' '\n' | '\n' | '\r';
Notice IDENTIFIER refers only to LETTERS.
If I provide this input:
a;
Then I get this error:
line 1:0 mismatched input 'a' expecting IDENTIFIER
(program a ;\n)
However if I uncomment the code and provide the same input I get legit output:
(program (statement a ;\n))
I do not understand why one works and the other does not.

Answer 1

The token a will only be assigned one token type. Since this input text matches both the LETTERS and IDENTIFIER rules, ANTLR 4 will assign the type according to the first rule appearing in the lexer, which means the input a will be a token of type LETTERS.
If you only meant for LETTERS to be a sub-part of other lexer rules, and not form LETTERS tokens themselves, you can declare it as a fragment rule.
fragment LETTERS : [a-z]+;
IDENTIFIER : LETTERS;
In this case, a would be assigned the token type IDENTIFIER and the original parser rule would work.