How \readlist* macro from listofitems package remove leading/trailing spaces

Question

This question follows How to remove space before or after argument then compare it with a string

The answer from @StevenB.Segletes use \readlist* macro which remove leading/trailing spaces from an item, can anyone help me understand how it is done. I took a look to listofitems.sty but I can't find where it is defined.

\documentclass{article}
\usepackage{listofitems}
\def\posa(#1){%
    \readlist*\myarg{#1}
    \if+\myarg[1]Plus\else \if-\myarg[1]Minus\else Retry \fi\fi
}   
\begin{document}

    \posa(+)

    \posa( -)

    \posa(?)

    \posa( + )

\end{document}