nodes of two borders will be either merged or inserted into faces of
the opposite border.
+In practice the borders to sew often coincide and in this case it is
+difficult to specify the first and the last nodes of a border since
+they coincide with the first and the last nodes of the other
+border. To cope with this, manually \ref merging_nodes_page to fuse
+each pair of coincident nodes into one.
+
The sewing algorithm is as follows:
<ol>
<li>The parameter (U) of each node within a border is computed. So
