[WJG] (17/April2006) It's often useful to keep lists of unique items but those lists need to be of fixed length. Adding a new item to the list, means that the oldest item  has to be deleted: a 'recent documents' list for example. Here's a simple solution. Indeed, its so simple that the comments are longer than the code itself!
 
 #---------------
 # lshift.tcl
 #---------------

 proc lshift {l i} {
  # if the item already exists, just return the list back
  if {![lsearch $l $i]} {return $l}
  
  # make a copy of the list with the new item at the top
  set j [linsert $l 0 $i]
  
  # return a list equalling the length of the original list sent
  set end [expr [llength $l] -1]
  return [lrange $j 0 $end]
 }

 #---------------
 # the ubiquitous demo
 #---------------
 proc demo {} {
  # just for the linux users
  catch {console show}

  # create a list
  foreach a {apple pear bannana} {
    lappend fruit $a
  }
 
  # the list before changes
  puts $fruit
 
  # cherry will be added, bannana will go
  set fruit [lshift $fruit cherry] 
  puts $fruit
 
  # we already have cherry, no change 
  set fruit [lshift $fruit cherry] 
  puts $fruit
  
  }
  
  demo