How to handle the mean of multiple files

[du hui <bjduhui@xxxxxxxxxxx>]

Dear ferret users: 
 I have sent a email about the question to how to do the mean of the 4 
files. But I think I made a mistake.  I have re-edited my script as 
following, but it doesn't work

use output.00.nc
use output.06.nc
use output.12.nc
use output.18.nc
set region/x=1:357.5/y=-90:90

let umean=(u[d=1,k=1]+u[d=2,k=1]+u[d=3,k=1]+u[d=4,k=1])/4
let vmean=(v[d=1,k=1]+v[d=2,k=1]+v[d=3,k=1]+v[d=4,k=1])/4
repeat/l=1:731 save/file=outputmean.nc/append umean,vmean

let umean=(u[d=1,k=2]+u[d=2,k=2]+u[d=3,k=2]+u[d=4,k=2])/4
let vmean=(v[d=1,k=2]+v[d=2,k=2]+v[d=3,k=2]+v[d=4,k=2])/4
repeat/l=1:731 save/file=outputmean.nc/append umean,vmean


When I excuted the above script, after finishing the level 1, then the 
following messages appeared,

let umean=(u[d=1,k=2]+u[d=2,k=2]+u[d=3,k=2]+u[d=4,k=2])/4
let vmean=(v[d=1,k=2]+v[d=2,k=2]+v[d=3,k=2]+v[d=4,k=2])/4
repeat/l=1:731 save/file=outputmean.nc/append umean,vmean
!-> REPEAT: L=1
LISTing to file outputmean.nc
**TMAP ERR: Requested data range is outside of data set limits
            Expected: UMEAN[K= 2 : 2 ] Found: UMEAN[K= 1 : 1 ]
LIST/FORMAT=CDF/file=outputmean.nc/append umean,vmean
Command file, command group, or REPEAT execution aborted

What is the mistake in my script?
Could anyone give me some hints for this problem
Thanks a lot

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