How to suppress ambiguity

To not have alternative for ambiguity in parse result there is two solution:

use unambiguous grammar
programmatocally rejecting rule match in match manager class

disambiguate grammar

The ambiguous grammar:

Start : Expression

Expression : 'a'
| Expression '+' Expression
| Expression '*' Expression

The disambiguate grammar ( operator precedence is managed at syntax level even if it is a semantic level concern )

Start : Expression

Expression : ExpressionMul
| Expression '+' ExpressionMul

ExpressionMul : PrimaryExpression
| ExpressionMul '*' PrimaryExpression

PrimaryExpression : 'a'

programmatically

rejecting rule match in match manager class, see match manager class.