  0,  2,  1,    2,  3,  1,   // left column
     4,  6,  5,    6,  7,  5,   // top run
     8, 10,  9,   10, 11,  9,   // middle run

    12, 13, 14,   14, 13, 15,   // left column back
    16, 17, 18,   18, 17, 19,   // top run back
    20, 21, 22,   22, 21, 23,   // middle run back

     0,  5, 12,   12,  5, 17,   // top
     5,  7, 17,   17,  7, 19,   // top rung right
     6, 18,  7,   18, 19,  7,   // top rung bottom
     6,  8, 18,   18,  8, 20,   // between top and middle rung
     8,  9, 20,   20,  