ead/3">head/3</a></code> is mainly intended for
<code>(<a href="refK.html#+Key">+Key</a> <a
href="refS.html#+String">+String</a>)</code> or <code>(<a
href="refR.html#+Ref">+Ref</a> <a href="refS.html#+String">+String</a>)</code>
relations,

<li><code><a href="refF.html#fold/3">fold/3</a></code> is useful for <code>(<a
href="refF.html#+Fold">+Fold</a> <a href="refR.html#+Ref">+Ref</a> <a
href="refS.html#+String">+String</a>)</code> relations,

<li><code><a href="refP.html#part/3">part/3</a></code> for <code>