pairs) can be added to the model one at a time or they
 can be provided in a 2-dimensional array.  The observations are not stored
 in memory, so there is no limit to the number of observations that can be
 added to the model.</p>
 <p>
 <strong>Usage Notes</strong>: <ul>
 <li> When there are fewer than two observations in the model, or when
 there is no variation in the x values (i.e. all x values are the same)
 all statistics return <code>NaN</code>. At least two observations with
 different x coordinates 