; docformat = 'rst'

;+
; Find the union of two sets of indices. A set of indices is represented by an
; array of non-negative integers where a scalar -1L indicates the empty set.
; 
; :Returns:
;    lonarr or -1L
;
; :Params:
;    ind1 : in, required, type=lonarr or -1L
;       array of indices where -1L indicates an empty set of indices
;    ind2 : in, required, type=lonarr or -1L
;       array of indices where -1L indicates an empty set of indices
;-
function mg_setunion, ind1, ind2
  compile_opt strictarr
  
  if (ind1[0] lt 0L) then return, ind2
  if (ind2[0] lt 0L) then return, ind1
  
  return, where(histogram([ind1, ind2], omin=omin)) + omin
end