the evaluation function part of a user defined expression.
 This function takes a label from a tree parser and returns a value or an error string.</p></div></div><div class="top"><p class="src"><a id="v:params" class="def">params</a> :: <a href="Expr.html#t:Expr" title="Expr">Expr</a> a -&gt; [<a href="Expr.html#t:Desc" title="Expr">Desc</a>] <a href="src/Expr.html#params" class="link">Source</a> <a href="#v:params" class="selflink">#</a></p><div class="doc"><p>params returns the descriptions of the paramet