[det]rb_insert(+Tree,
+Key, ?Value, -NewTree)Add an element with key Key and Value to the tree Tree
creating a new red-black tree NewTree. If Key is a
key in Tree, the associated value is replaced by Value.
See also rb_insert_new/4.
Does not validate that Key is sufficiently
instantiated to ensure the tree remains valid if a key is further
instantiated.