The constructor takes either the relangle-keyword or a
combination of relbulge and absbulge. The ``bulge'' is
meant to be a hint for the greatest distance between the connecting arc and the
straight connection between the box-centers. (Default: relangle=45,
relbulge=None, absbulge=None)
Note that the bulge-keywords override the angle-keyword.
If both relbulge and absbulge are given, they will be added.