function seq_dset, rmap, sequence, occurence, all=all
;
;+
;  Name: seq_dset
; 
;  Purpose: return dset vector corresponding to one occurence of 1 sequence
;  	table entry - uses serial number for grouping.  Function return
;  	value is used in rd_sda call. 
;
;  Parameters:
;	Input - rmap=roadmap structure
;		sequence=sxt sequence number
;		occurence=which occurence
;	Output- function returns data set vector
;
;  Calling Sequence:
;	dset = SEQ_DSET(rmap, sequence#, occurence 
;-
sequences=where(rmap.seq_num eq sequence)
if sequences(0) lt 0 then return, -1 		; error condition
if n_params() eq 2 then occurence=1
;
serials=rmap(sequences).serial_num		; find corresponding 
uniq_ser=serials(uniq(serials))			; uniqe serial numbers
occurencex=occurence < n_elements(uniq_ser)-1
;
; determine data set corresponding to occurence  
dset=sequences(where(rmap(sequences).serial_num eq uniq_ser(occurencex)))
;
return,dset
;
end