presto-parser/src/main/java/io/prestosql/sql/parser/AstBuilder.java/AstBuilder/visitSubstring
class AstBuilder:    @Override
    public Node visitSubstring(SqlBaseParser.SubstringContext context)
    {
        return new FunctionCall(getLocation(context), QualifiedName.of("substr"), visit(context.valueExpression(), Expression.class));
    }