I hope this will help others understand coroutines.  A special thanks to miguel on the #tcl freenode channel for taking the time to explain this to me.  Many of the words used here are his.  If there any errors, i have injected them.

===
proc allNumbers {} {
    yield
    set i 0
    while 1 {
      set howMuch [[yield $i]]
      incr i $howMuch
    }
}

coroutine nextNumber allNumbers

set theValue 2
for {set i 0} {$i < 10} {incr i} {
    puts "received [[nextNumber $theValue]]"
}

===

The first thing to understand is the [yield] command (which is in the coroutine's code).  [[yield $foo]] acts like [[return $foo]], but it suspends execution; when execution is resumed, the proc restarts on the line after the previous call to yield. ''MS notes that he kind of lied to jblz: [yield] is a bit more complicated, but behaves exactly as described for this example.''

[ZB] Wrong. It's the thing, that once confused me: the proc restarts not "on the line after previous call to yield", but resumes - one can say - directly in that lately left yield field, because there it can get ev. arguments "from outside". Consider the yield field as a "gate". You're leaving the proc - and getting back using this gate. Changed an example a bit - in my opinion, now it better explains the role of yield; changing theValue (and restarting the loop) one can see, how the increment step is different.

The procedure "allNumbers" is a proc which first calls [yield], and so suspends immediately (line 2); when it resumes it begins execution at line 3, after which it reaches another [yield] within a while loop on line 5.  After producing each number, it yields its value ... and suspends until it is called again.

Now this line:
===

coroutine nextNumber allNumbers

===
This line will (a) run allNumbers until the first [yield] (b) create a command called 'nextNumber' that resumes execution of allNumbers when nextNumber is called.

So after this we have allNumbers suspended right before 'set i 0', and a new command 'nextNumber' that continues it where it left off.  

Get it?