code: func toContracted(cn *cNode, lev uint) *mainNode {
  	if lev > 0 && len(cn.array) == 1 {
  		branch := cn.array[0]
  		switch branch.(type) {
  		case *sNode:
  			return entomb(branch.(*sNode))
  		default:
  			return &mainNode{cNode: cn}
  		}
  	}
  	return &mainNode{cNode: cn}
  }
docs: // toContracted ensures that every I-node except the root points to a C-node
  // with at least one branch. If a given C-Node has only a single S-node below
  // it and is not at the root level, a T-node which wraps the S-node is
  // returned.