VARIABLES
   Tcl allows the definition of variables and the use of their
   values either through $-style variable substitution, the set
   command, or a few other mechanisms.  Variables need not be
   declared: a new variable will automatically be created each
   time a new variable name is used.  Variables may be either
   global or local.  If a variable name is used when a
   procedure isn't being executed, then it automatically refers
   to a global variable.  Variable names used within a
   procedure normally refer to local variables associated with
   that invocation of the procedure.  Local variables are
   deleted whenever a procedure exits.  The global command may
   be used to request that a name refer to a global variable
   for the duration of the current procedure (this is somewhat
   analogous to extern in C).