The pick command is executed internally in response to pick
events (right mouse double click).
          
- COORDSYS
- = coordinate system in which coordinates of the following
arguments are specified.   This can be:
               
- world
- world coord sys
 
- self
- coord sys of the picked geom (GEOMID)
 
- primitive
- coord sys of the actual primitive within the picked geom where the pick
occurred. 
 
 
- GEOMID
-  = id of picked geom
 
- G
-  = picked point (actual intersection of pick ray with object)
 
- V
-  = picked vertex, if any
 
- E
-  = picked edge, if any
 
- F
-  = picked face
 
- P
-  = path to picked primitive [0 or more]
 
- VI
-  = index of picked vertex in primitive
 
- EI
-  = list of indices of endpoints of picked edge, if any
 
- FI
-  = index of picked face
External modules can find out about pick events by registering
interest in calls to pick via the interest command.
     
In the ND-viewing context the co-ordinates are actually ND-points. They
correspond to the 3D points of the pick relative to the sub-space
defined by the viewport of the camera where the pick occurred. The
co-ordinates are then padded with zeroes and transformed back to the
co-ordinate system defined by COORDSYS.