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The basic template is a quadrilateral with a pair of equal 
adjacent sides which 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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