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The basic template is a quadrilateral with two opposite 
sides which are equal and make equal and opposite angles 
with a glide reflection axis, taken here to be horizontal. 
The four  vertices are fixed within these constraints. The 
mid-points of the other two sides are then also fixed. The 
associated distortions allowed within each pair of related 
segments are indicated by the arrowheads, as are their 
rotational and glide reflective relations.
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