diff --git a/docs/pdf/Makefile b/docs/pdf/Makefile
index 2098749f..54862548 100644
--- a/docs/pdf/Makefile
+++ b/docs/pdf/Makefile
@@ -85,7 +85,7 @@
 
 TOPFILE   = userguide.tex
 SECFILE   = title.tex intro.tex commrout.tex datastruct.tex psbrout.tex toolsrout.tex\
-	methods.tex precs.tex
+	methods.tex precs.tex error.tex
 FIGDIR    = figures
 
 XPDFFLAGS = 
diff --git a/docs/pdf/error.tex b/docs/pdf/error.tex
new file mode 100644
index 00000000..e4f012a8
--- /dev/null
+++ b/docs/pdf/error.tex
@@ -0,0 +1,201 @@
+\section{Error handling}
+
+The PSBLAS library error handling policy has been completely rewritten
+in version 2.0. The idea behind the design of this new error handling
+strategy is to keep error messages on a stack allowing the user to
+trace back up to the point where the first error message has been
+generated. Every routine in the PSBLAS-2.0 library has, as last
+non-optional argument, an integer \verb|info| variable; whenever,
+inside the routine, en error is detected, this variable is set to a
+value corresponding to a specific error code. Then this error code is
+also pushed on the error stack and then either control is returned to
+the caller routine or the execution is aborted, depending on the users
+choice. At the time when the execution is aborted, an error message is
+printed on standard output with a level of verbosity than can be
+chosen by the user. If the execution is not aborted, then, the caller
+routine checks the value returned in the \verb|info| variable and, if
+not zero, an error condition is raised. This process continues on all the
+levels of nested calls until the level where the user decides to abort
+the program execution.
+
+Figure~\ref{fig:routerr} shows the layout of a generic \verb|psb_foo|
+routine with respect to the PSBLAS-2.0 error handling policy. It is
+possible to see how, whenever an error condition is detected, the
+\verb|info| variable is set to the corresponding error code which is,
+then, pushed on top of the stack by means of the
+\verb|psb_errpush|. An error condition may be directly detected inside
+a routine or indirectly checking the error code returned returned by a
+called routine. Whenever an error is encountered, after it has been
+pushed on stack, the program execution skips to a point where the
+error condition is handled; the error condition is handled either by
+returning control to the caller routine or by calling the
+\verb|psb\_error| routine which prints the content of the error stack
+and aborts the program execution.
+
+\begin{figure}[h!]
+  \begin{Sbox}
+    \begin{minipage}[tl]{0.95\textwidth}
+\small
+\begin{verbatim}
+subroutine psb_foo(some args, info)
+   ...
+   if(error detected) then
+      info=errcode1
+      call psb_errpush('psb_foo', errcode1)
+      goto 9999
+   end if
+   ...
+   call psb_bar(some args, info)
+   if(info .ne. zero) then
+      info=errcode2
+      call psb_errpush('psb_foo', errcode2)
+      goto 9999
+   end if
+   ...
+9999 continue
+   if (err_act .eq. act_abort) then
+     call psb_error(icontxt)
+     return
+   else
+     return
+   end if
+
+end subroutine psb_foo
+\end{verbatim}
+    \end{minipage}
+  \end{Sbox}
+  \setlength{\fboxsep}{8pt}
+  \begin{center}
+    \fbox{\TheSbox}
+  \end{center}
+  \caption{\label{fig:routerr}The layout of a generic \texttt{psb\_foo}
+  routine with respect to PSBLAS-2.0 error handling policy.}
+\end{figure}
+
+
+
+
+Figure~\ref{fig:errormsg} reports a sample error message generated by
+the PSBLAS-2.0 library. This error has been generated by the fact that
+the user has chosen the invalid ``FOO'' storage format to represent
+the sparse matrix. From this error message it is possible to see that
+the error has been detected inside the \verb|psb_cest| subroutine
+called by \verb|psb_spasb| ... by process 0 (i.e. the root process).
+
+
+\begin{figure}[h!]
+  \begin{Sbox}
+    \begin{minipage}[tl]{0.95\textwidth}
+\begin{verbatim}
+==========================================================
+Process: 0.  PSBLAS Error (4010) in subroutine: df_sample           
+Error from call to subroutine mat dist            
+==========================================================
+Process: 0.  PSBLAS Error (4010) in subroutine: mat_distv           
+Error from call to subroutine psb_spasb           
+==========================================================
+Process: 0.  PSBLAS Error (4010) in subroutine: psb_spasb           
+Error from call to subroutine psb_cest            
+==========================================================
+Process: 0.  PSBLAS Error (136) in subroutine: psb_cest            
+Format FOO is unknown
+==========================================================
+Aborting...
+\end{verbatim}
+    \end{minipage}
+  \end{Sbox}
+  \setlength{\fboxsep}{8pt}
+  \begin{center}
+    \fbox{\TheSbox}
+  \end{center}
+  \caption{\label{fig:errormsg}A sample PSBLAS-2.0 error
+    message. Process 0 detected an error condition inside the {\textrm
+    psb\_cest} subroutine}
+\end{figure}
+
+
+\subroutine{psb\_errpush}{Pushes an error code onto the error stack}
+
+\syntax{call psb\_errpush}{err\_c, r\_name, i\_err, a\_err}
+
+\begin{description}
+\item[\bf On Entry]
+\item[err\_c] the error code\\
+Scope: {\bf local} \\
+Type: {\bf required}\\
+Specified as: an integer.
+\item[r\_name] the soutine where the error has been caught.\\
+Scope: {\bf local} \\
+Type: {\bf required}\\
+Specified as: a string.\\
+\item[i\_err] addional info for error code\\
+Scope: {\bf local} \\
+Type: {\bf optional}\\
+Specified as: an integer array\\
+\item[a\_err] addional info for error code\\
+Scope: {\bf local} \\
+Type: {\bf optional}\\
+Specified as: a string.\\
+\end{description}
+
+\subroutine{psb\_error}{Prints the error stack content and aborts execution}
+
+\syntax{call psb\_error}{icontxt}
+
+\begin{description}
+\item[\bf On Entry]
+\item[icontxt] the communication context.\\
+Scope: {\bf global} \\
+Type: {\bf optional}\\
+Specified as: an integer.
+\end{description}
+
+
+
+\subroutine{psb\_set\_errverbosity}{Sets the verbosity of error messages.}
+
+\syntax{call psb\_set\_errverbosity}{v}
+
+\begin{description}
+\item[\bf On Entry]
+\item[v] the verbosity level\\
+Scope: {\bf global}\\
+Type: {\bf required}\\
+Specified as: an integer.
+\end{description}
+
+\subroutine{psb\_set\_erraction}{Set the type of action to be taken
+  upon error condition.}
+
+\syntax{call psb\_set\_erraction}{err\_act}
+
+\begin{description}
+\item[\bf On Entry]
+\item[err\_act] the type of action.\\
+Scope: {\bf global} \\
+Type: {\bf required}\\
+Specified as: an integer.
+\end{description}
+
+
+
+\subroutine{psb\_errcomm}{Error communication routine}
+
+\syntax{call psb\_errcomm}{icontxt, err}
+
+\begin{description}
+\item[\bf On Entry]
+\item[icontxt] the communication context.\\
+Scope: {\bf global} \\
+Type: {\bf required}\\
+Specified as: an integer.
+\item[err] the error code to be communicated\\
+Scope: {\bf global} \\
+Type: {\bf required}\\
+Specified as: an integer.\\
+\end{description}
+
+%%% Local Variables: 
+%%% mode: latex
+%%% TeX-master: "userguide"
+%%% End: 
diff --git a/docs/pdf/precs.tex b/docs/pdf/precs.tex
index b5c9ebaa..2eeb9404 100644
--- a/docs/pdf/precs.tex
+++ b/docs/pdf/precs.tex
@@ -1,6 +1,148 @@
 \section{Preconditioner routines}
 \label{sec:precs}
 
+% \section{Preconditioners}
+\label{sec:psprecs}
+Preconditioning is somehow regarded as ``black magic''. This is due to
+the fact that theory doesn't provide a reliable support in the choice
+of a preconditioner. It is clear that the influence of a
+preconditioning technique on the convergence behavior of an iterative
+method mostly depends on the characteristics of the system matrix and
+of the method itself. Anyway it is not possible a priori to say that
+one preconditioner is algebrically better than another and this perfectly explains
+the importance of providing a wide range of preconditioners techniques
+so that the user can find by itself which one is more suitable for his
+problem. Moreover, there are some other issues to consider when
+choosing a preconditioner such as balancing the overhead of building
+the preconditioner with the reduction in the number of
+iterations. PSBLAS contains the implementation of many preconditioning
+techniques some of which are very flexible thanks to the presence of
+many parameters that is possible to adjust to fit the user's needs:
+\begin{itemize}
+\item Diagonal Scaling
+\item Block Jacobi with ILU(0) factorization
+\item Additive Schwarz with the Restricted Additive Schwarz and
+  Additive Schwarz with Harmonic extensions (see
+  chapter~\ref{sec:pprec})
+\item Two-Level Additive Schwarz; this is actually a family of
+  preconditioners since there is the possibility to choose between
+  many variants as explained in chapter~\ref{sec:mlprec}
+\end{itemize}
+
+
+
+\subroutine{psb\_precset}{Sets the precodntioner type}
+
+\syntax{call psb\_precset}{prec, ptype, iv, rs, rv, ierr}
+
+\begin{description}
+\item[\bf On Entry]
+\item[prec] 
+Scope: {\bf global} \\
+Type: {\bf required}\\
+Specified as: e pronditioner data structure \precdata.
+\item[ptype] the type of preconditioner. 
+Scope: {\bf global} \\
+Type: {\bf required}\\
+Specified as: a string.
+\item[iv] integer parameters for the precondtioner. 
+Scope: {\bf global} \\
+Type: {\bf required}\\
+Specified as: an integer array.
+\item[rs] 
+Scope: {\bf } \\
+Type: {\bf }\\
+Specified as: .
+\item[rv] 
+Scope: {\bf } \\
+Type: {\bf }\\
+Specified as: .
+\item[ierr] 
+Scope: {\bf } \\
+Type: {\bf }\\
+Specified as: .
+\end{description}
+
+
+
+
+\subroutine{psb\_precbld}{Builds a preconditioner}
+
+\syntax{call psb\_precbld}{a, desc\_a, prec, info, upd}
+
+\begin{description}
+\item[\bf On Entry]
+\item[a] the system sparse matrix.
+Scope: {\bf global} \\
+Type: {\bf required}\\
+Specified as: a sparse matrix data structure \spdata.
+\item[desc\_a] the problem communication descriptor. 
+Scope: {\bf global} \\
+Type: {\bf required}\\
+Specified as: a communication descriptor data structure \descdata.
+\item[upd] 
+Scope: {\bf global} \\
+Type: {\bf optional}\\
+Specified as: a character.
+\end{description}
+
+\begin{description}
+\item[\bf On Return]
+\item[prec] the precodntioner.\\
+Scope: {\bf global} \\
+Type: {\bf required}\\
+Specified as: a precondtioner data structure \precdata\\
+\item[info] the return error code.\\
+Scope: {\bf local} \\
+Type: {\bf required}\\
+Specified as: an integer.\\
+\end{description}
+
+
+
+\subroutine{psb\_precaply}{Preconditioner application routine}
+
+\syntax{call psb\_precaply}{prec,x,y,desc\_a,info,trans,work}
+\syntax{call psb\_precaply}{prec,x,desc\_a,info,trans}
+
+\begin{description}
+\item[\bf On Entry]
+\item[prec] the preconditioner.
+Scope: {\bf global} \\
+Type: {\bf required}\\
+Specified as: a preconditioner data structure \precdata.
+\item[x] the source vector.
+Scope: {\bf global} \\
+Type: {\bf require}\\
+Specified as: a double precision array.
+\item[desc\_a] the problem communication descriptor.
+Scope: {\bf global} \\
+Type: {\bf required}\\
+Specified as: a communication data structure \descdata.
+\item[trans] 
+Scope: {\bf } \\
+Type: {\bf optional}\\
+Specified as: a character.
+\item[work] an optional work space
+Scope: {\bf local} \\
+Type: {\bf optional}\\
+Specified as: a double precision array.
+\end{description}
+
+\begin{description}
+\item[\bf On Return]
+\item[y] the destination vector.
+Scope: {\bf global} \\
+Type: {\bf required}\\
+Specified as: a double precision array.
+\item[info] the return error code.\\
+Scope: {\bf local} \\
+Type: {\bf required}\\
+Specified as: an integer.\\
+\end{description}
+
+
+
 %%% Local Variables: 
 %%% mode: latex
 %%% TeX-master: "userguide"
diff --git a/docs/pdf/userguide.tex b/docs/pdf/userguide.tex
index 7b71fd4c..4a27f2f5 100644
--- a/docs/pdf/userguide.tex
+++ b/docs/pdf/userguide.tex
@@ -90,7 +90,7 @@
 \include{toolsrout}
 \include{methods}
 \include{precs}
-
+\include{error}
 \end{document}
 %%% Local Variables: 
 %%% mode: latex