12     1   *** begin external enclosure 
    15     1
    15    22
     1    22
     1     3
     5     3
     5     4
     6     4
     6     5
     7     5
     7     6
     9     6
     9     5
    10     5
    10     4
    11     4
    11     2
    12     2
    12     1   *** end external grid enclosure