[W/K] :: tail recursion optimisation

  tail recursion optimisation
       
          (TRO) When the last thing a function or procedure does is to
          call itself, it is not necessary to retain the calling
          environment.  This is important when a procedure calls itself
          recursively many times for, without tail recursion
          optimisation, the environments of earlier invocations would
          fill up the memory only to be discarded when (if) the last
          call terminated.
       
          Tail recursion optimisation is a special case of last call
          optimisation but it allows the further optimisation that some
          arguments may be passed in situ, possibly in registers.  It
          allows recursive functions to be compiled into iterative
          loops.  See also conversion to iteration, tail recursion
          modulo cons.