For the man page, see http://www.purl.org/tcl/home/man/tcl8.4/TclCmd/append.htm
NAME append - Append to variable SYNOPSIS append varName ?value value value ...? DESCRIPTION Append all of the value arguments to the current value of variable varName. If varName doesn't exist, it is given a value equal to the concatenation of all the value arguments. This command provides an efficient way to build up long variables incrementally. For example, ``append a $b'' is much more efficient than ``set a $a$b'' if $a is long.
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