Sometimes an old link is provided, which is no longer work.
Example:
http://purl.org/thecliff/tcl/wiki/812.html

It should be changed to:
http://wiki.tcl.tk/812

Also, some links to Tcl core documentation changed file names
from .html to .md
Example:
https://core.tcl-lang.org/tips/doc/trunk/tip/407.md