Tracing Nested Tree Recursion

>(nnums '(+ (* x 3) (/ z 7)))
  1> (NNUMSB (+ (* X 3) (/ Z 7)) 0)
    2> (NNUMSB + 0)
    <2 (NNUMSB 0)
    2> (NNUMSB ((* X 3) (/ Z 7)) 0)
      3> (NNUMSB (* X 3) 0)
        4> (NNUMSB * 0)
        <4 (NNUMSB 0)
        4> (NNUMSB (X 3) 0)
          5> (NNUMSB X 0)
          <5 (NNUMSB 0)
          5> (NNUMSB (3) 0)
            6> (NNUMSB 3 0)
            <6 (NNUMSB 1)
            6> (NNUMSB NIL 1)
            <6 (NNUMSB 1)
          <5 (NNUMSB 1)
        <4 (NNUMSB 1)
      <3 (NNUMSB 1)
      3> (NNUMSB ((/ Z 7)) 1)
        4> (NNUMSB (/ Z 7) 1)
          5> (NNUMSB / 1)
          <5 (NNUMSB 1)
          5> (NNUMSB (Z 7) 1)
            6> (NNUMSB Z 1)
            <6 (NNUMSB 1)
            6> (NNUMSB (7) 1)
              7> (NNUMSB 7 1)
              <7 (NNUMSB 2)
              7> (NNUMSB NIL 2)
              <7 (NNUMSB 2)
            <6 (NNUMSB 2)
     ...
2