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### Validating User Inputs

Sometimes it may be necessary to check the user input against certain criteria. For example, you may want to allow only even numbers in a TixControl widget. To do this, you can use the `-validatecmd` option, which specifies a Tcl command to call whenever the user enters a new value. Here is an example:

`tixControl .c -value 0 -step 2 -validatecmd evenOnly
proc evenOnly {value} {
return [expr $value - ($value %2)]
}`

The value parameter to `evenOnly` is the new value entered by the user. The `evenOnly` procedure makes sure that the new value is even by returning a modified, even number. The Tcl command specified by the `-validatecmd` must return a value which it deems valid and this value will be stored in the `-value` option of the TixControl widget.