TRACK(R, X, I1, I2, K)

• R is a LAYOUT,
• X is an array of initial conditions and final upon return.
• K is an INTERNAL_STATE.
• Integer I1 is the position of the first FIBRE.
• I2 is the position of the second FIBRE: optional variable. PTC tracks all the way to the front of I2. If not present, the particle goes one turn or to the end of the beam line.