;+
;
; @file_comments
; find the union between 2 matrices of whole numbers
;
; see also <pro>different</pro> and <pro>inter</pro>.

; @categories
; Calculation
;
; @param a {in}{required}
; arrays of positive integers, which need
; not be sorted. Duplicate elements are ignored, as they have no
; effect on the result
;
; @param b {in}{required} 
; see a
;
; @returns
; an array containing the set of values in a and b.
;
; @restrictions 
; These functions will not be efficient on sparse sets with wide
; ranges, as they trade memory for efficiency. 
; The <proidl>HISTOGRAM</proidl> function
; is used, which creates arrays of size equal to the range of the
; resulting set.
;
; @examples
;
;   IDL> a = [2,4,6,8]
;   IDL> b = [6,1,3,2]
;   IDL> res=union(a,b) 
;   IDL> print, res
;           1           2           3           4           5           6
           8
; this is the list ao all elements in either sets.
;
; @history
; <a href="http://www.dfanning.com/tips/set_operations.html"/>
;
; @version
; $Id$
;
;-
FUNCTION union, a, b
;
  compile_opt idl2, strictarrsubs
;
IF a[0] LT 0 THEN RETURN, b    ;A union NULL = a
IF b[0] LT 0 THEN RETURN, a    ;B union NULL = b
RETURN, Where(Histogram([a,b], OMin = omin)) + omin ; Return combined set
END