code: function cossign(x, y, z) {
  			return (y[0] - x[0]) * (y[0] - z[0]) + (y[1] - x[1]) * (y[1] - z[1]);
  		}
docs: /* js/src/d2/pred/cossign.js 
  Computes the cosinus sign in a "right-handed"
  coordinate system (i.e. clockwise angle values).
  cossign(x, y, z) > 0 iff angle _xyz_ is acute.
  
  <p>
  
  http://en.wikipedia.org/wiki/Dot_product#Geometric_definition
  
  A.B = ||A|| ||B|| cos t
  A.B = A0 B0 + A1 B1
  A = [y[0] - x[0], y[1] - x[1]]
  B = [y[0] - z[0], y[1] - z[1]]
  A.B = (y[0] - x[0]) * (y[0] - z[0]) + (y[1] - x[1]) * (y[1] - z[1])
  
  
  # of operations
  
  - 1 add
  - 4 sub
  - 2 mul
  
  
  Originally implemented as
  
  return y[0] * (y[0] - x[0] - z[0]) +
  y[1] * (y[1] - x[1] - z[1]) +
  z[0] * x[0] +
  z[1] * x[1];
  
  with
  - 3 add
  - 4 sub
  - 4 mul