src/promissory-arbiter.js
function addChild (getValue, newChild, tree) {
    tree.children.splice(
      binaryIndexBy(getValue, getValue(newChild), tree.children),
      0, newChild
    );

    return newChild;
  }
function addChild (getValue, newChild, tree) {
    tree.children.splice(
      binaryIndexBy(getValue, getValue(newChild), tree.children),
      0, newChild
    );

    return newChild;
  }
function addChild ( getValue , newChild , tree ) { tree . children . splice ( binaryIndexBy ( getValue , getValue ( newChild ) , tree . children ) , 0 , newChild ) ; return newChild ; }
Adds a node child into the tree in order according to `getValue`
Adds a node child into the tree in order according to getValue