 to create frequently used envelope shapes:
      </p>

      <ul>
        <li>MAKE-LINEN</li>
        <li>MAKE-PERC</li>
        <li>MAKE-CUTOFF</li>
        <li>MAKE-ASR</li>
        <li>MAKE-ADSR</li>
        <li>MAKE-DADSR</li>
      </ul>

      <p>
        We can edit an existent envelope with
      </p>

      <ul>
        <li>EDIT-ENVELOPE</li>
        <li>SET-ENVELOPE-BASE</li>
        <li>(SETF ENVELOPE-LEVEL)</li>
        <li>(SETF ENVELOPE-TIME)</li>
        <li>(SETF ENVELOPE-CURVE)</li>
      