2dbsplinecurve b1 3 5 0 4 1 1 2 1 3 1 4 4  0 0 1  1 0.5 1   2 1 1  2 2 1  3 1.5 1  4 1.5 1   5 2 1 
2dcvalue b1 2.0 x y  dx dy 
set delta 0.005
point p1 x y 
set dyvalue [dval dy] 
repeat 100  {
	cmovetangent b1 2.0  x y dx $dyvalue 1
        set dyvalue [expr $dyvalue + $delta ]
        repaint }
copy b1 result
set length 5.9590472422107315