This does not seem to be a bug and the tutorial probably was not written
for GRASS5 with null data. If the layers are complementary and use nulls,
then the result will be nulls. It is necessary to change the nulls
in the complemenary files to zeroes (e.g. using r.null) to get it right.
I suggest that the tutorial should get info attached to it that it may contain
material which is not valid for GRASS5
Helena
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