fiftyfifty
fiftyfifty is a programming language and game
..., but by a binary tree. The part right of the head is represented by a binary tree, too. The head itself sort of sits between the two trees, but will often change its state according to information from the right side of the left side. In 5050, the state of the head itself is again represented as a tree of commands, although for simple programs, a linear representation can be sufficient.