I was wondering how you deincrement a FOR loop in tcl.  For example, if I have p = 10 and I want a FOR loop that keeps deincrementing p by 1 each time through the loop until p = 0.  I know about the incr command in FOR loops in tcl, but is there an opposite of this?

If you add an extra parameter onto the end of the incr command, that's the amount you increment by.  It can be any whole number, including -1 which will thus make it a decrement:
          incr  myvar  -1

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