Most menu entries may have additional self-explaining parameters, which may be checked. If some of these parameters are not available for some kind of action, they are disabled.

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Here is one example for executing an external program:

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You can see that a perl script is called, which should work with current selection. The Needs selected text parameter is checked to assure a selection. The output of this script will be inserted (Insert the output of the chosen program) and replace the current selection (Replace selected text), but not selected itself.