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tickbox variable: $revphaseflag -- reverse time axis
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default: 0 (FALSE)

The "rev" tickbox (just left of "angoff:" entry
on fn-F2 interface) reverse phase as well as real
valued color scales.

When revphaseflag is TRUE (revphase is selected),
the time axis is reversed, i.e. f(t) -> f(-t).
In general, this is not equivalent to multiplying
by -1.
