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(cross product for two vectors 1-2 and 3-2 having a common vertex 2)
(ccw ==> + result but note:)
(if traversing 1->2->3, a cw cross product ==> a ccw traverse)
(return:)
( _cross:z magnitude)
( _cross:dir 1 for ccw per convention)
o<cross> sub
(not_a_subfile)
#<x1> = #1
#<y1> = #2
#<x2> = #3
#<y2> = #4
#<x3> = #5
#<y3> = #6
#<_cross:z> = [[#<x1>-#<x2>]*[#<y3>-#<y2>]-[#<x3>-#<y2>]*[#<y1>-#<y2>]]
#<_cross:dir> = 0 (default: 0)
o<l10> if [#<_cross:z> GT 0]
#<_cross:dir> = 1 (ccw)
o<l10> endif
o<l20> if [#<_cross:z> LT 0]
#<_cross:dir> = -1 (cw)
o<l20> endif
o<line> call [#<x2>][#<y2>][#<x1>][#<y1>]
#<len21> = #<_line:len>
o<line> call [#<x2>][#<y2>][#<x3>][#<y3>]
#<len23> = #<_line:len>
(print cross: #<x1> #<y1> #<x2> #<y2> #<x3> #<y3> #<x4> #<y4>)
(print cross: #<x1> #<x2> #<x3>)
(print cross: #<y1> #<y2> #<y3>)
(print cross: z=#<_cross:z> dir=#<_cross:dir>)
(print )
(debug cross: z=#<_cross:z> dir=#<_cross:dir>)
o<cross> endsub
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