; docformat = 'rst'
;+
;This function combines either two rotational matrices or two quaternions.
;The two inputs must agree in their type (either they are both matrices or
;they are both quaternions).
;
; :Examples:
;   mat_com = combine_rotation(mat1, mat2)
;-
;+
; :Params: 
;   T1 : in, required, type=array  of double
;     Either a 9 element rotational matrix or a 4 element quaternion of 
;     the form [3D vector, scalar].
;   T2 : in, required, type=array of double
;     Either a 9 element rotational matrix or a 4 element quaternion of
;     the form [3D vector, scalar].
;     
; :Returns:
;   Returns either a combined rotational matrix or a combined quaternion.
;-
function combine_rotation,T1,T2
  if(n_elements(T1) ne n_elements(T2)) then begin
    message,/error,'Two transformations of different type'
    return,-1
  end

  if(n_elements(T2) eq 9) then begin
    ;It's a matrix
    return,T2 ## T1
  end else begin
    ;It's a quaternion
    return,quat_mult(T1,T2);
  end
end