t.cpp(5): CAT(A,  B)
  t.cpp(2): see macro definition: CAT(a, b)
  invoked with
  [
    a = A
    b =  B
  ]
  [
    CAT_(A, B)
    rescanning
    [
      t.cpp(3): see macro definition: CAT_(a, b)
      invoked with
      [
        a = A
        b =  B
      ]
      [
        AB
        rescanning
        [
          AB
        ]
      ]
      AB
    ]
  ]
t.cpp(5): AB(X,  Y)
  t.cpp(4): see macro definition: AB(x, y)
  invoked with
  [
    x = X
    y =  Y
  ]
  [
    CAT(X, Y)
    rescanning
    [
      t.cpp(2): see macro definition: CAT(a, b)
      invoked with
      [
        a = X
        b =  Y
      ]
      [
        CAT_(X, Y)
        rescanning
        [
          t.cpp(3): see macro definition: CAT_(a, b)
          invoked with
          [
            a = X
            b =  Y
          ]
          [
            XY
            rescanning
            [
              XY
            ]
          ]
          XY
        ]
      ]
      XY
    ]
  ]