[R] adding variable to netCDF file

[Cable, Samuel B Civ USAF AFMC AFRL/RVBXI]

I would like to open an existing netCDF file and add a variable to it.
I am using the ncdf package.  This test code gives the idea of what I am
trying to do:

library(ncdf)

print('here we go')

print('first, construct netCDF file')
t<-dim.def.ncdf('t','',1:1,unlim=T,create_dimvar=F)
print('defining first vars')
a<-var.def.ncdf('a','km',t,missval=-999)
b<-var.def.ncdf('b','km',t,missval=-999)

print('creating ncobj')
ncobj<-create.ncdf('testfile.nc',list(a,b))


print('adding data')
put.var.ncdf(ncobj,a,c(1,2,3,4,5),count=5)
put.var.ncdf(ncobj,b,c(10,20,30,40,50),count=5)

print('file constructed.  close it.')
close(ncobj)

print('defining new var')
c<-var.def.ncdf('c','hrs',t,missval=-999)

print('now open file back up for adding variable')
ncobj<-open.ncdf('testfile.nc',write=T)

print('adding new var to ncobj')
ncnewer<-var.add.ncdf(ncobj,c)

print('new data')
put.var.ncdf(ncnewer,c,c(.1,.2,.3,.4,.5))

close.ncdf(ncnewer)

Everything works fine until the call to var.add.ncdf(), where R stops
with the very helpful error message "This is not allowed."  Anybody see
what I am doing wrong?  Note that I am indeed calling open.ncdf() with
write=T, as needed.  Thanks.