Pasctran is an ancient cult. [1] Some say they're extinct. Others say its practitioners have just gone underground, now that society will no longer tolerate such things done in public. It's everywhere once you start looking though, insidiously contaminating our precious function bodies.

From the source to the original Bourne shell:

    BEGIN
       REG BOOL slash; slash=0;
       WHILE !fngchar(*cs)
       DO    IF *cs++==0
        THEN    IF rflg ANDF slash THEN break; ELSE return(0) FI
        ELIF *cs=='/'
        THEN    slash++;
        FI
       OD
    END

[1] https://research.swtch.com/shmacro