[[explain importance of this notion
of immutability to proper
understanding of Tcl's semantics]]

Those who have a little knowledge of C (or other similar languages) have learned
to pass arguments to a function ''by value'' or ''by reference''.

Tcl scripts have such functionalities : when you need to pass the variable ''by value'',
you put a '''$''' before the variable name. And this ''does not modifiy'' the variable.

But when you need to access uplevel variables, you can use [upvar] in the proc you call,
passing just the name of the variable as argument. Then you can access the content of a
variable defined in the upper stack level.
This feature works especially good with [array]s that cannot be passed by value.

But when you come to Tcl C API (the internals that are required to write C extensions),
you must know how Tcl passes variables by value, which makes use of the ''copy-on-write'' semantics.

----


[[and consequences for extension design]]

   ???