Optional argument of [if]. It's usually good style to omit the
''then'' itself, but can aid clarity when using multi-line
expressions. Compare with [else].

 if {$some_long_and_complex_condition
     && $some_other_condition_making_it_even_longer
 } then {
      # do the right thing here
 } else {
      # do the other right thing here
 }


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[Category Command] | [Category Control Structure]