requires function

[Pikaqiu]

I have an issue with my robot code.
"Function 'requires' could not be resolved."

#include "toggleDown.h"
#include "Robot.h"
#include "RobotMap.h"

toggleDown::toggleDown() {
// Use requires() here to declare subsystem dependencies
// eg. requires(chassis);
// BEGIN AUTOGENERATED CODE, SOURCE=ROBOTBUILDER ID=REQUIRES
requires(Robot::elevator);
// END AUTOGENERATED CODE, SOURCE=ROBOTBUILDER ID=REQUIRES
}

Where can I find the function definition of requires?

I built my code with robotbuilder.

Please help,
Thanks.