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I am working on a corporate installation. They have emacs, and it has tcl mode. The issue is that the command lassign is not highlighted, unlike, for example, set, or return.

What that I would like, is that w/o changing the original, installed, Tcl-Mode ELISP code, add lassign (and others) to the list/dict that are highlighted.

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You can do this using the function font-lock-add-keywords. It takes two arguments, the first is the major mode to extend. Alternatively, you can pass nil which mean the current buffer, which is useful if you call the function from a hook. The second is a list of font-lock keywords. This is a special data structure described in the documentation of the variable font-lock-keywords.

For example, it could look like:

(defun my-c-mode-hook ()
  (font-lock-add-keywords nil
    '(("\\_<\\(k[A-Z0-9][a-zA-Z0-9_]*\\)\\_>" (0 font-lock-constant-face)))))
(add-hook 'c-mode-hook #'my-c-mode-hook)

The font-lock keyword match words like kThisIsAConstant using a regular expression (or regexp). The 0 mean that the entire match should be highlighted and font-lock-constant-face is used to highlight the match

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