How to do the mean of 4 files

[du hui <bjduhui@xxxxxxxxxxx>]

Hi,dear ferret users
 I have four netcdf files which have the same variables, same level, same 
data range, but different time.  ie, one data file is observed at 00 
o'clock, the other three are 06, 12, 18 respectively.
 What I want is to mean these 4 time's data and get a new data which can 
be called mean.nc
I have write the following scirpt whick works well

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

************************************
Because I want to compute all the levels and all the date range
so I add the following two lines 
repeat/k=1:2
repeat/l=1:731

But this time it doesn't work

Can someboday give some guidence for this problem and how can get what I 
want

Thanks a lot

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