; docformat = 'rst'
;+
; Evaluates the cross product of vector a with vector b. The input
; vectors must have 3 elements.
;
; :Author:
;   DMS: 08-1983,
;   Chris Jeppesen: 07-2005 (to multiply a whole grid),
;   Ryan Talley: 06-08-2015 (added rst formatting and error test)
;
; :Examples:
;   product = crossp_grid(vector_a, vector_b)
;-
;+
; :Params:
;   a : in, required, type=vector of int or vector of double
;     Input vector of three elements used to compute the cross product.
;   b : in, required, type=vector of int or vector of double
;     Input vector of three elements used to compute the cross product.
;
; :Returns:
;   Returns the cross product that is a 3-element, floating-point vector.
;-
Function crossp_grid,a,b

    resolve_grid,a,x=ax,y=ay,z=az
    resolve_grid,b,x=bx,y=by,z=bz
 
    x=ay*bz-az*by
    y=az*bx-ax*bz
    z=ax*by-ay*bx

    return,compose_grid(x,y,z)
end