This command treats the variable given by varName as a list
and appends each of the value arguments to that list as a separate
element, with spaces between elements.
If varName doesn't exist, it is created as a list with elements
given by the value arguments.
Lappend is similar to append except that the values
are appended as list elements rather than raw text.
This command provides a relatively efficient way to build up
large lists. For example, ``lappend a $b'' is much
more efficient than ``set a [concat $a [list $b]]'' when
$a is long.