How can I mainpulate the arguments in a function defined with \fp_new_function:n?

I just discovered that we can now create new functions (fp words) that can be evaluated inside \fp_eval:n per page 273 of interface3.pdf (it also seems to work with \fpeval). This is straightforward as the demo shows.

What if I want, or need, to manipulate the function arguments to get the returned result? I can't get that to work. This functionality may not be provided yet, but I don't see it mentioned in the documentatin and searching this site turned up nothing (or I somehow missed it).

MWE:

% !TEX program = lualatexmk
% !TEX encoding = UTF-8 Unicode

\documentclass{article}

\ExplSyntaxOn
% This works.
\fp_new_function:n { npow }
\fp_set_function:nnn { npow } { a,b } { a**b }

% This does not work.
\fp_new_function:n { biggerof }
\fp_set_function:nnn { biggerof } { a,b } 
  {
    \fp_compare:nTF { a < b }
    { b }
    { a }
  }
\ExplSyntaxOff

\begin{document}
\( \alpha = \fpeval{npow(2,3)} \)

\( \alpha = \fpeval{biggerof(2,3)} \)
\end{document}