From Tcl 8.5, list non-inclusion [expr] operator. E.g.
 if {$el ni $list} ...
Opposite of [in]. - [rs] 2007-04-26



 set el Mon
 set list [list Mon Tue Wed Thu Fri Sat Sun]
 if {$el ni $list} {
   puts "Invalid day abbreviation"
 } else {
   puts "Valid day abbreviation"
 }

'''TeXies beware!''' This is not a \ni (∋), but a \notin (∉).

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