The hypercircle through the vertices is associated with the hyperbolic line XY.
For each i, C(i) is the perpendicular projection fro A(i) to XY.
Then we have saccheri quadrilaterals A(i)A(i+1)C(i+1)C(i), (i = 1..4)
and A(5)A(1)C(1)C(5). The sides A(i)C(i) each have hyperbolic length d,
the width of the hypercircle.
