+<p>
+An existing MASK raster map is respected for both reading <em>input</em>
+and writing <em>output</em>.
is that a newly added feature or documenting an old one?
if new, does it help speed up the very slow searches where there
is no input data? (e.g. when the region contains a large lake or
partial ocean, where there are no contour lines)
+<p>
+An existing MASK raster map is respected for both reading <em>input</em>
+and writing <em>output</em>.
is that a newly added feature or documenting an old one?
An old one (absent in grass 3.2, introduced in grass 4.0), but it was
not working properly in grass6 because of missing NULL support, unless
a MASK used zero and not NULL for masking.
if new, does it help speed up the very slow searches where there
is no input data? (e.g. when the region contains a large lake or
partial ocean, where there are no contour lines)
Yes, it does help speed up (now that NULL cells in a MASK are skipped
as intended).
Apart from floating point support, I have not added any new features
to r.surf.contour.