

    \filetitle{difflog}{First log-difference pseudofunction}{modellang/difflog}

	\paragraph{Syntax}

\begin{verbatim}
difflog(EXPR)
difflog(EXPR,K)
\end{verbatim}

\paragraph{Description}

If the input argument \texttt{K} is not specified, this pseudofunction
expands to

\begin{verbatim}
(log(EXPR)-log(EXPR{-1}))
\end{verbatim}

If the input argument \texttt{K} is specified, it expands to

\begin{verbatim}
(log(EXPR)-log(EXPR{K}))
\end{verbatim}

The two derived expressions, \texttt{EXPR\{-1\}} and \texttt{EXPR\{K\}},
are based on \texttt{EXPR}, and have all its time subscripts shifted by
--1 or by \texttt{K} periods, respectively.

\paragraph{Example}

The following two lines of code

\begin{verbatim}
difflog(Z)
difflog(X{1}/Y{-1},-2)
\end{verbatim}

will expand to

\begin{verbatim}
(log(Z)-log(Z{-1}))
(log(X{1}/Y{-1})-log(X{-1}/Y{-3}))
\end{verbatim}


