From Tcl 8.5 [http://wiki.tcl.tk/10630], [expr] operator for list inclusion test.
 if {$el in $list} ...
is equivalent to, but much nicer than
 if {[lsearch -exact $list $el] >= 0} ...
The negation ("not in") is [ni]. So 8.5 Tcl really makes us knights who say 'ni'...
For users of earlier versions, the following little helper comes in handy:
 proc in {list el} {expr [lsearch -exact $list $el] >= 0}}
 ...
 if [in $list $el] ...


- [rs] 2007-04-26

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Let's see - so it would be something like:

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


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[Category Command] , well sub-command, as part of expr