'a t is the type of a variable holding a value of type 'a

create x creates a variable with initial value x

create_tracking node creates a variable that tracks the value of node. Such a variable behaves as a normal variable except that:

• it is initialized to the value of node at creation time
• when the value of node changes, the variable is reset to that of node.

peek v returns the value currently held in the variable.

set v x updates the variable v to hold the value x. This automatically invalidates all nodes that depend directly and indirectly on v.

set_with_undo v x behaves as set. However, it returns a value of type undo_info holding information allowing to revert the effect of the assignement.