function testSplit_Case4() {
  var tree = insertToTree(23);
  var expected =
      '11[13|27]\n' +
      '_{2|20|12}_\n' +
      '2[5|10]  20[15|21]  12[31|45]\n' +
      '_{0|13|15}11  2{1|17|3}11  20{5|7|9}11\n' +
      '0[1|3]  13[5|9]  15[10|11|12]  1[13|14]  17[15|16|17]  3[21|22|23|25]' +
      '  5[27|29]  7[31|38]  9[45|47|49]\n' +
      '_{1/3}2  0{5/9}2  13{10/11/12}2' +
      '  15{13/14}20  1{15/16/17}20  17{21/22/23/25}20' +
      '  3{27/29}12  5{31/38}12  7{45/47/49}12\n';
  assertEquals(expected, tree.toString());
}
Split of leaf node causing double promotion.

                                27
              /                                        \
         5|10|13|21                                  31|45
 /      /    |           \            \          /     |      \
1|3  5|9  10|11|12  13|14|15|17  21|22|23|25  27|29  31|38  45|47|49

insert 16

                             13|27
              /                |                           \
     5|10                    15|21                       31|45
  /   |      \         /       |          \         /      |       \
1|3  5|9  10|11|12  13|14  15|16|17  21|22|23|25  27|29  31|38  45|47|49