How to remove items from a list not by their position ([lreplace]) but by the actual content.
My solution would be:

 proc K { x y } { set x }
 proc lremove { listvar string } {
 	upvar $listvar in
 	foreach item [K $in [set in [list]]] {
 		if {[string equal $item $string]} { continue }
 		lappend in $item
 	}
 }
which gives us

 % set a [list a b c a b c a b c]
 a b c a b c a b c
 % lremove a b
 % set a
 a c a c a c

Is there any reason not to do it that way?

Gotisch