Availability (open-source, closed-source, or commercial): open-source with binary
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From the website
TrEd is a fully customizable and programmable graphical editor and viewer for tree-like structures. Among other projects, it was used as the main annotation tool for syntactical and tectogrammatical annotations in The Prague Dependency Treebank, as well as for decision-tree based morphological annotation of The Prague Arabic Dependency Treebank.