1. Numeric primitives
	- coord: unitless coordinate type
	- calc: calculations with coordinates are done using this type; normally wider than coord
	- offs: to express in-object offset values [0..1]

2. Low level primitives
	- vector: a coord;coord pair
	- box: a rectangle specified with min-x;min-y and max-x;max-y vectors;
	       typically used for bounding boxes
	- cap: pen shape: round (circle) or square
	- stroke: a cap and size (in coord)

3. Centerline objects (thin, non-stroked, 0-width objects)
	- cline: a line segment specified by its two endpoints
	- ccirc: a circle specified by a center point vector and radius
	         (the only reason for this type is optimization for speed in
	         some internal calculations)
	- carc: an arc (segment of a circle) specified by a center point vector,
	  a radius, a start angle and a delta angle

4. Stroked objects
	- sline: specified by a cline and a stroke
	- sarc: specified by a carc and a stroke

5. Polygon
A polygon is always specified by centerline-tpye (non-stroked, zero width)
outlines.
TODO