function find_common, x, y

;
;  check input 
;
if n_params() lt 2 then begin
   print,'Use: c = find_common(first, second)'
   return, -1
endif

;
;  vectors only
;
if (size(x))(0) ne 1 or (size(y))(0) ne 1 then begin
   print,'Input parameters must be vectors.'
   return, -1
endif

;
;  search for occureences of the values of the first vector
;
n = -1
for i=0L,n_elements(x)-1 do begin
   n = [n,where(y eq x(i))]
endfor
nn = where(n ge 0)

;
;   if none, clean exit
;
if nn(0) eq -1 then return,-1

;
;  else return indices in second vector
;
n = n(nn)
n = n(sort(n))
return, n(rem_dup(n))

end