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<artificial> undefined reference to class::method( )

Can someone help me figure out why this is happening ? My directory structure follows the format in the readmes in lib and include. Namely, all my headers are in src and I include them in the relevant classes. So I am not sure what is going wrong, and would appreciate help.

I have 2 classes, Wheel and Robot.
Robot has Wheels as properties:

Robot.h

class Robot{
    public:
    Robot();
    private:
    Wheel frontWheels[2]; //instantiate a wheel obj. using default c-tor
    Wheel backWheels[2]; //instantiate a wheel obj. using default c-tor
    
    public:
    void turn(int angle);
    void moveFwd();
    void moveBackwd();
    
    private:
    //

};
#endif

Robot Constructor:

Robot::Robot(){
    //instantiate front wheels
    for (int i = 0; i < 2; i ++){
        Wheel w = Wheel(22 + i, 23 + i, 2+i);
        this->frontWheels[i] = w;
    }
    for (int i = 0; i < 2; i ++){
        Wheel w = Wheel(26 + i, 27 + i, 4+i);
        this->backWheels[i] = w;
    }
}//end contsructor

main.cpp

#include <Arduino.h>
#include "Robot.h"

Robot u0;
void setup() {
  Serial.begin(9600);
}

void loop() {
  u0.moveFwd();
  delay(1000);
  u0.moveBackwd();

}

Wheel:
the wheel class just sets up the infrastructure for motors. it has been unit tested and verified.

#include "Wheel.h"

Wheel::Wheel(uint p1, uint p2, uint enable){
    pinMode(p1, OUTPUT); 
    pinMode(p2, OUTPUT);
    pinMode(enable, OUTPUT);
    this->pin1 = p1; this->pin2 = p2; this->enable = enable;
}

void Wheel::turnFwd(){
    digitalWrite(this->pin1, HIGH);
    digitalWrite(this->pin2, LOW);
}

void Wheel::turnBackwd(){
    digitalWrite(this->pin2, HIGH);
    digitalWrite(this->pin1, LOW);
}

In which file is this code located?