I'm not having any luck using an `\ifx` command inside a `\DeclareFieldFormat` block with biblatex (v1.6). Is there something I'm missing? A minimal (non-)working example:

```
\documentclass{article}
\usepackage{filecontents}
\begin{filecontents}{\jobname.bib}
@misc{ref1,
    title = {Title1},
    eprint = {MR001},
    eprinttype = {mrnumber}}
@misc{ref2,
    title = {Title},
    eprint = {002},
    eprinttype = {mrnumber}}
@misc{ref3,
    title = {Title},
    eprint = {MR003 (aa)},
    eprinttype = {mrnumber}}
@misc{ref4,
    title = {Title},
    eprint = {004 (bb)},
    eprinttype = {mrnumber}}
\end{filecontents}
\usepackage{biblatex}
\addbibresource{\jobname.bib}

% based on code from http://tex.stackexchange.com/q/1856/245
\def\checkMR MR#1#2#3 #4\relax%
  {\ifx#1M%
     \ifx#2R%
     #3%
     \else%
     #1#2#3%
     \fi
   \else
     #1#2#3%
   \fi}
\def\MR#1{\checkMR MR#1 \relax}

\DeclareFieldFormat{eprint:mrnumber}{%
  \MR{#1}}

\begin{document}
It works here \MR{MR001}, \MR{002}, \MR{MR003 (aa)}, \MR{004 (bb)}

\cite{ref1}, \cite{ref2}, \cite{ref3}, \cite{ref4}
\printbibliography
\end{document}

```

It evaluates correctly in the body, but as false in the bibliography field.