What is the meaning of #+: and #-: in common lisp
<p>I am reading the <a href="https://github.com/edicl/cl-fad/blob/master/load.lisp" rel="nofollow">cl-fad/load.lisp</a> code tonight, and I found there are symbols <code>#+:</code> and <code>#-:</code> in the front of expression or string. </p>

<p>What's these symbols meaning? </p>