Combinations of the same particles: $\pi$$\sp$+ $\pi$$\sp$+

       C---First select pion candidates
              DO 5 ITK=KFCHT,KLCHT
              IF(condition to select pions) THEN
                ISAVE=KIDSAV(ITK,'PI+')
              ENDIF
            5 CONTINUE
       C---Loop over selected pions.
              IPIONE = KPDIR ('PI+', KRECO)
           10 IF (IPIONE .NE. 0)  THEN
                IPITWO = KFOLLO (IPIONE)                 <--- important
           20   IF (IPITWO .NE. 0)  THEN
                  ... analysis of the pi+ pi+ system ...
                  IPITWO = KFOLLO (IPITWO)
                  GO TO 20
                ENDIF
              IPIONE = KFOLLO (IPIONE)
              GO TO 10
              ENDIF

The 2nd $\pi$$\sp$+ (IPITWO) has to be initialized with KFOLLO and NOT with KPDIR. See section 9.2.11 for the use of KIDSAV.