\section{Previous work}

One of the first implementations of the \commonlisp{} \texttt{loop}
macro is the one that is often referred to as ``MIT LOOP''
\cite{Burke:Moon:MIT.loop}.

\ecl{} uses MIT LOOP with only minor modifications.

\sbcl{} includes an implementation of the \texttt{loop} macro that was
originally derived from MIT LOOP, but that also includes code from the
\texttt{loop} macro of the \genera{} operating system.  Furthermore,
the \sbcl{} implementation of the \texttt{loop} macro has been
modified and improved to allow for user-definable extensions, such as
the extension defined in \cite{Rhodes:2007:USC:1622123.1622138} for
iterating over user-definable sequences.

\clisp{} has its own implementation of the \texttt{loop} macro.  The
bulk of the implementation can be found in a function named
\texttt{expand-loop}.  This function consists of more than $900$ lines
of code.

\section{Previous work}

One of the first implementations of the \commonlisp{} \texttt{loop}
macro is the one that is often referred to as ``MIT LOOP''
\cite{Burke:Moon:MIT.loop}.

\ecl{} includes two implementations of the \texttt{loop} macro, namely
the initial MIT LOOP with only minor modifications, and the variation
by \symbolics{} also with minor modifications.

\sbcl{} includes an implementation of the \texttt{loop} macro that was
originally derived from MIT LOOP, but that also includes code from the
\texttt{loop} macro of the \genera{} operating system.  Furthermore,
the \sbcl{} implementation of the \texttt{loop} macro has been
modified and improved to allow for user-definable extensions, such as
the extension defined in \cite{Rhodes:2007:USC:1622123.1622138} for
iterating over user-definable sequences.

\clisp{} has its own implementation of the \texttt{loop} macro.  The
bulk of the implementation can be found in a function named
\texttt{expand-loop}.  This function consists of more than $900$ lines
of code.

Add paragraph explaining that ECL has two loop implementations.
Add paragraph explaining that ECL has two loop implementations.
