; Calculates the separation angle, in radians, between two vectors.
; 
; Inputs: a, b are the vectors in question.  
; 
; IMPORTANT: The first dimension of both factors must be 3!

function separation_angle, a, b
  return, acos( total (a*b, 1 )/(sqrt( total (a^2,1))*sqrt( total (b^2,1))))
  end