-- extracts the 3 columns centered at the given ppm frequency
-- requires iNMR version 2.4.6
function x3ct( ppm )
index = transpose()				-- the f-index along x
spec = getf( index )			-- the paramters of the spectrum
if (ppm < spec.start) then
	print "too low a frequency"
	return false
end
if (ppm > spec.start + spec.width) then
	print "too high a frequency"
	return false
end
one = spec.width / spec.size	-- digital resolution in ppm units
mark( ppm - one, ppm, ppm + one )
extract()
return true
end