[mjk]: Here is a simple example of using the [after] for tail-recursive calls. The following example calculates Fibonacci numbers without exhausting the stack.

 proc _fib {i a b} {
     if {$i <= 2} then {
 	set ::_fibretval [expr $a + $b]
     } else {
 	after 0 _fib [expr $i - 1] $b [expr $a + $b]
     }
 }
 
 proc fib {i} {
     after 0 _fib $i 0 1
     vwait ::_fibretval
 
     return $::_fibretval
 }
 
 puts [fib  1] ;# => 1
 puts [fib  2] ;# => 1
 puts [fib  3] ;# => 2
 puts [fib 10] ;# => 55
 puts [fib 30] ;# => 832040
 puts [fib 46] ;# => 1836311903