Arc length of a path

In this example we use the `arclen` method of the path instance `p1` to get the
arc length and create a straight line `p2` with the same arc length. ...

! While here we use the return value of the arc length as a parameter to a path
constructor, you might be interested in what kind of object the return value of
the `arclen` method call really is: It is a PyX length in true coordinates,
since the path already analyses the PyX units and the return value thus needs
to be fixed and can not be scaled anymore. Printing the result of the `arclen`
method call of path `p1` would show you:

  (0.023110 t + 0.000000 u + 0.000000 v + 0.000000 w + 0.000000 x) m
