function mapSingleChildIntoContext(bookKeeping, child, childKey) {
  var result = bookKeeping.result,
      keyPrefix = bookKeeping.keyPrefix,
      func = bookKeeping.func,
      context = bookKeeping.context;


  var mappedChild = func.call(context, child, bookKeeping.count++);
  if (Array.isArray(mappedChild)) {
    mapIntoWithKeyPrefixInternal(mappedChild, result, childKey, emptyFunction.thatReturnsArgument);
  } else if (mappedChild != null) {
    if (ReactElement.isValidElement(mappedChild)) {
      mappedChild = ReactElement.cloneAndReplaceKey(mappedChild,
      // Keep both the (mapped) and old keys if they differ, just as
      // traverseAllChildren used to do for objects as children
      keyPrefix + (mappedChild.key && (!child || child.key !== mappedChild.key) ? escapeUserProvidedKey(mappedChild.key) + '/' : '') + childKey);
    }
    result.push(mappedChild);
  }
}
This is the abstract API for an update queue.