 phase by a regression of pressure on scan number.  There are three
variants to this, depending on the value of the second <code>method</code> element.
If <code>method</code> is <code>"A"</code> (or not given), the procedure is to
call <code><a href="https://rdrr.io/r/stats/nls.html" class="external-link">nls()</a></code> to fit a piecewise linear model of pressure as a function of scan,
in which pressure is constant for scan less than a critical value, and then
linearly varying for with scan. This is  mean