presto-parser/src/main/java/io/prestosql/sql/parser/AstBuilder.java/AstBuilder/visitDistinctFrom
class AstBuilder:    @Override
    public Node visitDistinctFrom(SqlBaseParser.DistinctFromContext context)
    {
        Expression expression = new ComparisonExpression(
                getLocation(context),
                ComparisonExpression.Operator.IS_DISTINCT_FROM,
                (Expression) visit(context.value),
                (Expression) visit(context.right));

        if (context.NOT() != null) {
            expression = new NotExpression(getLocation(context), expression);
        }

        return expression;
    }