http://www.purl.org/tcl/home/man/tcl8.4/TclCmd/lset.htm

----
Kevin Kenny writes on news:comp.lang.tcl.announce :

 Subject: TIP's 22, 33, 45 - extended list functionality - now final.
 Date: 18 Nov 2001 11:30:33 -0800
 Message-ID: <pgpmoose.200111181130.19875@non.non.net>

Jeff Hobbs today committed to the CVS HEAD at Sourceforge the changes
described in TIP's 22 (Multiple Index Arguments to lindex), 33 (Add 'lset'
Command to Assign to List Elements), and 45 (Empty index lists for [lindex]
and [lset])

These changes augment the [lindex] command so that it can extract elements
from sublists, for example:

    [lindex {{a b c} {d e f} {g h i}} 1 1] => e

They also implement an [lset] command that may be used to change individual
elements within lists and sublists.  Taken together, these commands can be
used to treat lists as if they were linear arrays.  For instance, the
following procedure might be used to reverse the order of elements in a
list.

    proc reverse { list } {
        set i 0
        set j [expr { [llength $list] - 1 }]
        while { $j > $i } {
            set temp [lindex $list $i]
            lset list $i [lindex $list $j]
            lset list $j $temp
            incr i
            incr j -1
        }
        return $list
    }

Updated documentation for the commands is available in the 'doc/' 
subdirectory in the source tree.  The original proposals may be found at

    http://www.purl.org/tcl/tip/22.htm 
    http://www.purl.org/tcl/tip/33.htm 
    http://www.purl.org/tcl/tip/45.htm