syntax problem with r.mapcalculator

I created a raster map, "R20101001", using v.surf.rst.  The interpolation 
overshoots and dips below zero in a few areas.  I'd like to use 
r.mapcalculator, called from spgrass6, to replace all negative values with 
zeros.  But I can't seem to get the syntax right.  The following fails and 
produces the error message: "The system cannot find the file specified."

execGRASS("r.mapcalculator", flags = c("overwrite"),
  parameters=list(outfile="result", amap="R20100101", 
formula='if(A<0,0,A)'))

I would appreciate it if anyone can spot the mistake.

library(spgrass6)
data(meuse.grid)
coordinates(meuse.grid) <- c("x", "y")
gridded(meuse.grid) <- TRUE
fullgrid(meuse.grid) <- TRUE
meuse.grid$ddev <- meuse.grid$dist - mean(meuse.grid$dist, na.rm=TRUE)
loc <- initGRASS("/home/rsb/topics/GRASS/g640/grass-6.4.0",
   home=tempdir(), SG=meuse.grid)
writeRAST6(meuse.grid["ddev"], "ddev")
execGRASS("r.mapcalculator", flags = c("overwrite"),
   parameters=list(outfile="result", amap="ddev", formula="'if(A<0,0,A)'"))
res <- readRAST6("result")
summary(res)
summary(meuse.grid["ddev"])

works for me under Linux with: formula="'if(A<0,0,A)'", protecting the 
inner quotes with outer quotes. On Windows the incantation will be 
different, possibly:

execGRASS("r.mapcalculator", flags = c("overwrite"),
   parameters=list(outfile="result", amap="ddev",
     formula="\"if(A<0,0,A)\""))

(untried on Windows, also works on Linux).

Hope this helps,

Roger

PS. The statsgrass list has more direct access to GRASS users and 
developers with experience of shell quoting solutions.

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Roger Bivand
Economic Geography Section, Department of Economics, Norwegian School of
Economics and Business Administration, Helleveien 30, N-5045 Bergen,
Norway. voice: +47 55 95 93 55; fax +47 55 95 95 43
e-mail: Roger.Bivand at nhh.no