Return the item that comes after item in the container. Only makes sense for sorted containers. Raises an element-not-found-error if the item isn't present in the container.
| successor < metabang.cl-containers:binary-search-tree > < metabang.cl-containers::bst-node > |
| successor < metabang.cl-containers:container-uses-nodes-mixin > < t > |
| successor < metabang.cl-containers:dlist-container > < metabang.cl-containers:dlist-container-node > |