or calculates the <a href="http://en.wikipedia.org/wiki/Arithmetic_mean">arithmetic mean</a> of a sequence of values. </li>    <li>source and selector can&#39;t be null, and are validated immediately. </li>    <li>The operators all use <em>immediate execution</em>. </li>    <li>The operators all iterate over the entire input sequence, unless an exception is thrown (e.g. due to overflow). </li>    <li>The operators with a non-nullable return type throw InvalidOperationException if the input sequence is empty