code: function SHAMapTreeNodeInner(depth) {
    SHAMapTreeNode.call(this);
    this.leaves = {};
    this.type = SHAMapTreeNode.INNER;
    this.depth = depth === undefined ? 0 : depth;
    this.empty = true;
  }
docs: Inner (non-leaf) node in a SHAMap tree.
  @param {Number} depth (i.e. how many parent inner nodes)
  @class