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The basic template is a hexagon, having one pair of equal and 
opposite sides which are parallel. The two sides at each end of these 
make equal pairs, and within each pair the sides make equal and 
opposite angles with the line joining their unshared ends, taken here 
to be horizontal. The six vertices are fixed within these constraints. 
The associated distortions allowed within each related pair of 
segments are indicated by the arrowheads, as are their tanslational 
and glide reflective relations.
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