psgml
Emacs is an advanced and extensible editor. An Emacs major mode customizes Emacs for editing particular types of text documents. PSGML is a major mode for SGML (a markup language) documents. PSGML provides several functionalities for editing SGML documents: indentation according to element nesting depth and identification of structural errors (but it is not a validating SGML parser); menus and commands for inserting tags with only the contextually valid tags; editing attribute values in a separate window with information about types and defaults; structure based editing, including movement and killing; and also several commands for folding editing.
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