export defaultfunction(callback) {
for (var groups = this._groups, j = 0, m = groups.length; j < m; ++j) {
for (vargroup = groups[j], i = 0, n = group.length, node; i < n; ++i) {
if (node = group[i]) callback.call(node, node.__data__, i, group);
}
}
returnthis;
}