Locations, angles, etc.

 Key type is a `loc', which is a named location (absolute, 2D) and
 direction (at the location).  Angles are generally positive
 anticlockwise; bearings are angles measured from East.  The origin is
 at the South-West.

Syntaxes
 New location
  <identifier>
 Existing location
  [-]<identifier>  (- means reverse the sense)
 Quantity (length/angle)
  <number>[<unit>]
  Angle units: d (degrees,default)  r (radians)
  Length units: mm (default)  cm  m
 Identifiers (of locs and objects) start with lc letter, then
 alphanums and underscores

Commands

 abs P X Y A
  Defines loc P given coordinates (distances) and absolute bearing

 rel F T L R A
  Defines loc T: start at loc F, go forward L, translate right R,
  turn left A.

 extend F T len L [R]        length L
 extend F T upto U [R]       s.t. perpendicular at T passes through U
 extend F T ang A R          subtending directionally A
 extend F T uptoang A R      s.t. direction at T is A
 extend F T parallel U R     s.t. direction at T is same as at U
  Draws an arc or line from loc F, defining the other end as loc T.
  If length R specified, draws an arc of radius R; R +ve curves to the
  right; R -ve to the left.

 join F T R [S ...]
  Joins one existing loc, F, to another, T.  F's direction points to
  the new track; T's away - ie the added track leaves F in F's
  direction and arrives at T in T's direction.  R is the minimum curve
  radius allowed.  S selects from the available solutions, and may be
  any of the following
     long                  prefer longer length solution
     short                 prefer shorter length solution
     right|left            prefer mostly bending to the left resp. right
     beginright|beginleft  prefer first bend to the right
     endright|endleft      prefer final bend to the right
     [!]twoarcs            prefer [not] two circular arcs of equal radius
     [!]arcline            prefer [not] one line and an arc of max radius
     [!]arcsline           prefer [not] line between two arcs of min radius
  if this doesn't resolve, will pick the shortest.

 defobj O
 [commands]
 enddef
  Defines the object O.  Inside the definition, the commands do not
  draw when the object is being defined.  The object has its own
  coordinate space and its own location namespace.

 obj[flip] O A F [P]
  Places an instance of object O.  The loc defined inside O as F
  (`formal parameter', F must be just <identifier>) is placed at
  existing loc A (`actual parameter').  If prefix P (syntax is that of
  an identifier) is specified, each other loc L inside O is exported
  into the global namespace as a new loc PL.  If `flip' is used then
  the object is placed with object-space y coordinates negated (ie, it
  is mirrored so that the object's North exchanges with South).