A Question About \futurelet

The \let and \futurelet primitives create implicit character tokens. Thus when does

\futurelet\foo\baz a

the token \foo is an a. It is not a macro which expands to an a.

One can see this by using \show and \meaning:

\def\baz{\edef\test{\meaning\foo}\show\foo\show\test}
\futurelet\foo\baz a

You can do anything with \foo that doesn't require an explicit character token. For example, if we do

\let\bgroup={

we can do

\hbox\bgroup <content>

but not

\def\foo\bgroup <definition>

One can use \meaning to disect the text, for example if we know it's a letter

\def\baz{%
  \edef\test{\meaning\foo}%
  \edef\test{\expandafter\bazaux\test\stop}%
  \show\test
}
\edef\bazaux{\def\noexpand\bazaux\detokenize{the letter }##1\noexpand\stop{##1}}
\bazaux
\futurelet\foo\baz a

(One could of course test first for this case.)

My LaTeX colleague, Christian Tellechea, as I was asking him about the limitations of implicit character tokens, quickly shot back a macro (using the listofitems package) that he calls \implicittomacro#1 that takes an implicit token (limited to catcode 11 or 12) and turns it into a macro.

Thus, the MWE I posted at the end of my question becomes do-able with Christian's help. In it, the macro \fltest uses \futurelet to capture the undigested next token, and then can test it against a multiplicity of match characters at once (here, both x and y). This is cooler than a mere \ifx test, which can only compare against a single token at a time.

\documentclass{article}

\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{listofitems}
\def\implicittomacro#1{%
\def\next{\expandafter\implicittomacroi\meaning#1\implicittomacroi#1}%
     \unless\ifcat a\noexpand#1%
         \unless\ifcat.\noexpand#1%
             \def\next{\errmessage{\string#1\space is not catcode 11 or 
12}}%
         \fi
     \fi
     \next
}
\expandafter\def\expandafter\implicittomacroi\expandafter
#\expandafter1\detokenize{er} #2\implicittomacroi#3{%
\def\implicittomacroii##1\implicittomacroii{\endgroup\def#3{##1}}%
     \begingroup\endlinechar-1\everyeof{\noexpand}%
\expandafter\implicittomacroii\scantokens{#2\implicittomacroii}%
}

\def\fltest{\futurelet\xfl\pdecide}
\def\pdecide{%
  \implicittomacro\xfl%
  \setsepchar{x||y}% SEARCH FOR x or y AS THE NEXT CHARACTER
  \readlist\mylist{\xfl}%
  \ifnum\listlen\mylist[]>1\relax%
    (Following undigested token is: \mylistsep[1])%
  \else
    (Following undigested token is NEITHER x nor y)%
  \fi
}
\begin{document}
\fltest xyz

\fltest yzx

\fltest zxy
\end{document}