public Builder add(SubDocument subDocument) {
Preconditions.checkState(!build, "This builder has already been used");
SubDocType type = subDocument.getType();
Integer index = indexBySubDoc.get(type);
if (index == null) {
index = 0;
} else {
index++;
}
indexBySubDoc.put(type, index);
subDocuments.put(type, index, subDocument);
return this;
}
