The icons in this folder were taken from the Tango desktop project
http://tango.freedesktop.org/Tango_Desktop_Project

According to their web site, the icons are Public Domain:
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Terms of Use?

The color palette is public domain. The icon theme is also available as public domain since
the 0.8.90 release. While you are not obligated to, we would appreciate if you credit the
project if you chose to use the icon theme or derivative artwork in your project by linking
to our website. This will help to strengthen the awareness of the style guidelines. Thank you.
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Icons included from Tango:
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dialog-error.png
dialog-information.png
dialog-warning.png
document-open.png
document-properties.png
document-save.png
edit-clear.png
edit-redo.png
edit-undo.png
emblem-important.png
go-bottom.png
go-down.png
go-first.png
go-home.png
go-jump.png
go-last.png
go-next.png
go-previous.png
go-top.png
go-up.png
help-browser.png
list-add.png
list-remove.png
process-stop.png
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Please note that you can obtain the icons in higher quality (including SVG format) as well
as additional icons at the Tango desktop project web site!