local variables are now created automatically when they are first assigned to.
the global keyword can be used to declare that a name refers to a global variable instead,
at which point, assigning to the name results in the assignment referencing the global variable
instead of allocating a new global variable.
the var keyword allows greater control over what scope a
particular variable exists in. it clarifies whether a new
variable is being defined or an existing variable is being
assigned to. it will also facilitate the implementation of
global variables.
iteration is implementing without recursion, instead using type-specific callbacks to construct a queue of nodes to iterate through. ast priting is implemented using this functionality.
