docs: public InternalDocumentRevision lookupChildByRevId(InternalDocumentRevision parentNode, String childRevision) {
          Misc.checkNotNull(parentNode, "Parent node");
          Misc.checkArgument(sequenceMap.containsKey(parentNode.getSequence()),
                  "The given parent DocumentRevision must be in the tree.");
  
          DocumentRevisionNode p = sequenceMap.get(parentNode.getSequence());
          Iterator i = p.iterateChildren();
          while(i.hasNext()) {
              DocumentRevisionNode n = (DocumentRevisionNode) i.next();
              if(n.getData().getRevision().equals(childRevision)) {
                  return n.getData();
              }
          }
          return null;
      }