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Negative concentrations can arise from explicit mixing/advection in tracer and
from high-latitude filtering (option O_fourfil) in filtr.  I could not figure
out how to modify the mixing/advection and Fourier-filtering algorithms so that
no negative concentrations are created.  So I check for negative concentrations
and redistribute the affected tracer in the surrounding regions (see also the
explanations in subroutines filt and tracer).