\begin{thebibliography}{2}
\expandafter\ifx\csname natexlab\endcsname\relax\def\natexlab#1{#1}\fi
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  \def\bibnamefont#1{#1}\fi
\expandafter\ifx\csname bibfnamefont\endcsname\relax
  \def\bibfnamefont#1{#1}\fi
\expandafter\ifx\csname citenamefont\endcsname\relax
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\providecommand{\bibinfo}[2]{#2}
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\bibitem[{car({\natexlab{a}})}]{carpet-carpetweb}
\emph{\bibinfo{title}{Mesh refinement with {Carpet}}},
  \urlprefix\url{http://www.carpetcode.org/}.

\bibitem[{car({\natexlab{b}})}]{carpet-cactusweb}
\emph{\bibinfo{title}{{Cactus} {Computational} {Toolkit}}},
  \urlprefix\url{http://www.cactuscode.org/}.

\end{thebibliography}