`proc` and `func` uses static dispatch. To enable dynamic dispatch, `method`
must be used instead:
[https://nim-lang.org/docs/manual.html#multiminusmethods](https://nim-lang.org/docs/manual.html#multiminusmethods)
This appears to be working:
type
BBTree*[K,V] = BBLeaf[K, V] # BBTree is a generic type with keys and
values of types K, V
## `BBTree` is an opaque immutable type.
BBLeaf[K,V] = ref object of RootObj
key: K # the search key; must support the generic
``cmp`` proc
val: V # the data value associated with the key, and
stored in a node
BBNode[K,V] = ref object of BBLeaf
vleft: BBTree[K,V] # left subtree; may be nil
vright: BBTree[K,V] # right subtree; may be nil
vsize: int # the size of the (sub-)tree rooted in this
node
method size[K,V](n: BBLeaf[K,V]): int = return 1
method left[K,V](n: BBLeaf[K,V]): BBTree[K,V] = discard
method right[K,V](n: BBLeaf[K,V]): BBTree[K,V] = discard
method size[K,V](n: BBNode[K,V]): int = return n.vsize
method left[K,V](n: BBNode[K,V]): BBTree[K,V] = return n.vleft
method right[K,V](n: BBNode[K,V]): BBTree[K,V] = return n.vright
Run
