QUEUE OF AVAILABLE ELEMENTS         

         +--------+                   
I.QHDR:  |   Q3   +--+
         +--------+  |
                     |
Q1:      +--------+  |
      +--+   Q2   |<-+
      |  +--------+
      |  |        |
      |  |        |
      |  +--------+
      |
Q2:   |  +--------+
      +->|   Q3   +--+
         +--------+  |
         |        |  |
         |        |  |
         +--------+  |
                     |
Q3:      +--------+  |
         |   0    |<-+
         +--------+
         |        |
         |        |
         +--------+