

    \filetitle{diff}{First difference pseudofunction}{modellang/diff}

	\paragraph{Syntax}
 
 \begin{verbatim}
 diff(EXPR)
 diff(EXPR,K)
 \end{verbatim}
 \paragraph{Description}
 
 If the input argument \texttt{K} is not specified, this pseudofunction
 expands to
 
 \begin{verbatim}
 ((EXPR)-(EXPR{-1}))
 \end{verbatim}
 If the input argument \texttt{K} is specified, it expands to
 
 \begin{verbatim}
 ((EXPR)-(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}
 
 These two lines
 
 \begin{verbatim}
 diff(Z)
 diff(log(X{1})-log(Y{-1}),-2)
 \end{verbatim}
 will expand to
 
 \begin{verbatim}
 ((Z)-(Z{-1}))
 ((log(X{1})-log(Y{-1}))-(log(X{-1})-log(Y{-3})))
 \end{verbatim}


