Ok, here is a slightly refined Path API, as exposed
through IDL:

typedef sequence<Vertex> Polyline;
struct Quadric {/*...*/};
struct Bezier
{
    unsigned short   order;
    sequence<Vertex> controls;
};
struct Nurbs
{
    unsigned short   order;
    sequence<Vertex> controls;
    sequence<double> weights;
    sequence<double> knots;
};

enum PathSegmentType { polylinetype, arctype, beziertype, nurbstype};

union PathSegmentContent switch (PathSegmentType)
{
   case polylinetype: Polyline lines;
   case arctype:      Quadric  arc;
   case beziertype:   Bezier   bezier;
   case nurbstype:    Nurbs    nurbs;
};

//. moveto: the next segment's CS is the same as this,
//.         end points are not connected
//.
//. lineto: the next segment's CS is the same as this,
//.         end points are connected by a line
//.
//. align: the next segment's CS is this plus a translation
//.        that aligns the last point of this segment with
//.        the first point of the next segment
enum PathSegmentConnection { moveto, lineto, align};

struct PathSegment
{
   PathSegmentContent    segment;
   //. connection defines the connection to the next
   //. segment. If this is the last one, moveto implies
   //. an open path, lineto a closed path. Align is
   //. undefined...
   PathSegmentConnection connection;
};

typedef sequence<PathSegment> Path;