Subtract track momenta

Subtract the vector ISUB from ISUM and store the result in IDIFF. Space for IDIFF can be reserved by IDIFF = KVNEW (DUMMY).

• If QE(ISUM) < QE(ISUB), the result is meaningless.
• If QP(IDIFF) > QE(IDIFF), the result gets a negative mass.
• A warning is issued in either case.