http://www.purl.org/tcl/home/man/tcl8.4/TclCmd/lset.htm
will eventually have the formal man page for lset.
----
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 [Element]s), 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 [array]s.  For instance, the
following [proc]edure 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