Blurb::
Trend function order of the model discrepancy

Description::

The order of the trend function may be specified using the \c trend_order
command, followed by either 0, for which a constant trend function will
be used, 1, for which a linear trend function will be used, or 2, for which
a quadratic trend function will be used.

<b> Default Behavior </b>

By default, a quadratic trend function is used.
