For a set <i> S</i> = {<i>a, b, c, </i> ...} let    <i> S<sup>-1</sup></i> =
 {<i>a<sup>-1</sup>, b<sup>-1</sup>, c<sup>-1</sup>,... </i>}.    
A <i> word</i> is a  formal finite string of the form
<i> x<sub>1</sub>x<sub>2</sub>x<sub>3</sub></I>, ... 
where <BR><i>x<sub>i</sub>&#8712 S &#8746 S<sup>-1</sup></I>.