Starting with the Tcl 8.5 release, the variable varName passed to incr may be unset, and in that case, it will be set to the value increment or to the default increment value of 1. If varName indicate an element that does not exist of an array that has a default value set, the sum of the default value and the increment (or 1) will be stored in the array element.
Add 42 to the contents of the variable x:
incr x 42
Add the contents of the variable y to the contents of the variable x:
incr x $y
Add nothing at all to the variable x (often useful for checking whether an argument to a procedure is actually integral and generating an error if it is not):
incr x 0