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     Will run the rest of the line as a system command.

     Will interpret the rest of the line as a comment.

     Will pop the top two values off of the stack.  The old
     second-to-top value will be stored in the array R, indexed by the
     old top-of-stack value.

     Pops the top-of-stack and uses it as an index into the array R.
     The selected value is then pushed onto the stack.