#define MRR       14  // L293D right motor control lead
#define MRB       15  // L293D right motor control lead
#define MLR       16  // L293D left  motor control lead
#define MLB       17  // L293D left  motor control lead

stop_all()
{
   pinhigh(MRR);
   pinhigh(MRB);
   pinhigh(MLR);
   pinhigh(MLB);
}

drive(ml, mr, dly)
// use the polarities of mr and ml to control the turning directions of motors
//  1 = forward
// -1 = backward
//  0 = stop
{
   if(ml  > 0) { pinlow(MLB); pinhigh(MLR); } else if(ml < 0) { pinhigh(MLB); pinlow(MLR); }
   if(mr  > 0) { pinlow(MRR); pinhigh(MRB); } else if(mr < 0) { pinhigh(MRR); pinlow(MRB); }
   if(dly > 0) delay(dly); 
}