lappend − Append list elements onto a variable
lappend varName ?value value value ...? ______________________________________________________________________________
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 does not 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.
Using lappend to build up a list of numbers.
% set var 1
% lappend var 2
% lappend var 3 4 5
1 2 3 4 5
append, element, list, variable