NAME
Swap — Swap the values of two variables.
SYNOPSIS
Swap VarNameA VarNameB
DESCRIPTION
Return Value
None.
Arguments
VarNameAFirst variable to swap value of.
VarNameBSecond variable to swap value of.

For information regarding exceptions / errors, see here.
EXAMPLES 
% set X 111
111
% set Y 222
222
% Swap X Y
% puts "X = $X, Y = $Y"
X = 222, Y = 111
KEYWORDS
modify, variable