On facebook someone asked "how do you stop tcl script at a particular line"

The quickest way, if you know the line is (immediately before that line):

======
error "I have to stop here"
======

But if you are looking for something a little more elegant, create a proc which tells Tcl to enter the event loop, and stay there until a condition has been satisfied. In this case we will assume our program is running from a command line, and we want to prompt the user to press any key to continue.

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proc breakpoint {} { 
  update
  set ::breakpoint 0
  puts stdout "Press <Any Key> to continue"
  fileevent stdin readable {set ::breakpoint 1}
  vwait ::breakpoint
}
======

For a tk based application:
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proc breakpoint {} { 
  update
  tk_messageBox -type ok -message "Press OK to continue"
}
======
----
[RS] 2014-05-08 My favorite breakpoint, which allows interacting with a running process like with a debugger, is this:
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proc interact args {
    while 1 {
        puts -nonewline "$args% "
        flush stdout
        gets stdin cmd
        if {$cmd=="exit" || $cmd=="c"} break
        catch {uplevel 1 $cmd} res
        if [string length $res] {puts $res}
    }
}
======
The zero or more arguments are displayed before the % prompt, so if you have more than one of these breakpoints, you can name them:
 interact here
 ...
 interact there
or, to display in which proc the breakpoint is located:
 interact [info level 0]
An earlier attempt is at [A minimal debugger].

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