expr : expr '+' ident | ident

void ShiftIdent() {
    current_token++;
}

void ShiftPlus() {
    current_token++;
}

void ShiftExpr() {
    switch(current_token->type) {
    case ident:
        ShiftIdent();
        break;
    default:
        error();
    }
    while(true) {
        switch(current_token->type) {
        case '+':
            ShiftPlus();
            ShiftIdent();
            break;
        default:
            return;
        };
    }
}