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http://rosettacode.org/wiki/100_doors
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■ HundredDoors
  § static
    ▶ main
    • args⦂ String[]
      open⦂   boolean: true
      closed⦂ boolean: false
      doors⦂  boolean[1+100] · all initially closed
      🔁 pass from 1 to 100
         ∀ visited ∈ pass‥100 by pass
         · toggle the visited doors
           if the doors[visited] are closed
              let the doors[visited] be open
           else
              let the doors[visited] be closed
      for each door #n in doors⦂ boolean
        if the door is open
           System.out.println "Door #⸨n⸩ is open."