(dot product for two vectors 1-2 and 3-2 having a common vertex 2)
(note u=[Ucos[a],Usin[a]], v=[V[cos[b],Vsin[b] then:)
(     u dot v = U*Vcos[a-b]    phi := a-b        )
(     u dot v > 0 ==> acute    phi  = [-90, 90]  )
(     u dot v < 0 ==> obtuse   phi  = [ 90,270]  )
(     u dot v = 0 ==> right    phi  = 90         )

(return:)
( _dot:dot scalar product)
(_dot:ang  [   0,180] angle between vectors -- for cw,ccw determination)
(_dot:phi  [-180,180] actual angle between vectors)
(_dot:acu  [   0, 90] acute angle between the extended vectors)

o<dot> sub
(not_a_subfile)
#<x1> = #1
#<y1> = #2
#<x2> = #3
#<y2> = #4
#<x3> = #5
#<y3> = #6

#<dx1> = [#<x1>-#<x2>]
#<dy1> = [#<y1>-#<y2>]
#<dx3> = [#<x3>-#<x2>]
#<dy3> = [#<y3>-#<y2>]

#<_dot:dot> = [#<dx1>*#<dx3> + #<dy1>*#<dy3>]
#<magp>     = SQRT[[#<dx1>*#<dx1>+#<dy1>*#<dy1>]*[#<dx3>*#<dx3>+#<dy3>*#<dy3>]]
#<_dot:ang> = ACOS[#<_dot:dot>/#<magp>]

        #<_dot:phi> = #<_dot:ang>
o<dot0> if [#<_dot:dot> LT 0]
          #<_dot:phi> = [ 0 - #<_dot:ang>]
o<dot0> endif

        #<_dot:acu> = #<_dot:phi>
o<dot1> if [#<_dot:ang> GT 90]
          #<_dot:acu> = [180  - #<_dot:ang>]
o<dot1> endif

(print  dot: #<x1> #<y1> #<x2> #<y2> #<x3> #<y3> magp=#<magp> )
(print  dot: dot=#<_dot:dot> ang=#<_dot:ang> phi=#<_dot:phi> acu=#<_dot:acu>)

o<dot> endsub
