Slur with two turning points
A slur normally can only have one turning point. In special situations, one may need two or more turning points, however. This can be achieved by linking two (or more) slur stencils together. The control points need to be specified manually: for two segments this is a set of seven points, of which the fourth is shared by both segments. To ease working with the function, the control points are shown as light blue crosses if the second argument is ##t.
For a more refined solution to the problem, see Shaping Bezier Curves, needing version 2.19.48 or higher.