Not a Problem
in 3D:
creat a line with points A,B
type point(f,0)
move A or B
does the point(f,0) must be undefined?
The same problem 
in 3D:
creat a line with points A,B
type point(f,0)
move A or B
does the point(f,0) must be undefined?
OOH! now I see. point(f,0) is an 'extreme' of line, so it is undefined. but if you type point(f,0) the created point is a central point
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