t containing the unit is returned (or <code>NULL</code> if there is no such unit).
This list consists of an item named <code>unit</code>, which is an <code><a href="https://rdrr.io/r/base/expression.html" class="external-link">expression()</a></code>, and
an item named <code>scale</code>, which is a string describing the measurement scale.
If the string ends in <code>" unit"</code>, e.g.  <code>x[["temperature unit"]]</code> (note the
space), then just the expression is returned, and if it ends in <code>" s