Since version XXX two important features have been added: compression and
a better handling of multiple weights.

In order to compile a process with this version, the Makefile
should be modified as follows:
   A line LIBS+=-lz should be inserted before the USER= ... line;
   The PWHG variable should be augmented by adding lhefread.o,
   pwhg_io_interface.o, rwl_weightlists.o and rwl_setup_param_weights.o;
The development files of zlib should be available (the zlib1g-dev package
on Ubuntu). It is available by default on CERN platforms. A dummy zlib
package is included in the distribution. By linking it, everything works
but without compression.

You need a late gfortran compiler, 5.1.0 and 5.2.1 are known to work.
On lxplus at CERN you can do
 source /afs/cern.ch/sw/lcg/contrib/gcc/5.2/x86_64-slc6-gcc52-opt/setup.sh
and recompile.

COMPRESSION

In order to activate compression of les Houches files, insert:

compress_lhe 1

in the poweg.input file. The generated .lhe file will be gz compressed.
You can examine it by using zmore, zless or zcat. Alternatively, doing
$ mv pwgevents.lhe pwgevents.lhe.gz
$ gzip -d pwgevents.lhe.gz
to get back the ascii file. The compression factor is roughly 5.

The flag acts only for writing files. When reading files (in order to add
new weights, for example) the io interface detects whether the file is
compressed or not, and reads it in all cases.

When using the storemintupb flag, it is often userful to compress also the 



NEW WEIGTHS HANDLING

The new weight system is added to the previous one, and is fully independent upon it. The older system
will be removed at some point. It is activated by inserting in the powheg.input file the line

rwl_file 'filename'

where filename is the name of the file containing the xml description of the weights. Alternatively, one
can use

rwl_file '-'

<initrwgt>
...
</initrwgt>

and insert the xml weight description in the powheg.input file itself. The <initrwgt> and </initrwgt> lines
must be alone in a line. The weights are inserted according to the standard described in arXiv:1405.1067.
POWHEG should set its parameter according to the description present in each weight. For example
<initrwgt>
<weightgroup name='PDFS'>
<weight id='1'> lhapdf=10800 </weight>
<weight id='2'> lhapdf=10801 </weight>
</weightgroup>
<weight id='3'> lhapdf=10802 facscfact=0.5 </weight>
</initrwgt>

The interpretation of the settings for each weight is performed by the subroutine in rwl_setup_param_weights.f.
It looks for keywords=value pairs in the description field of the weight, and it performs the appropriate
action. At the moment it only handles PDF's and scale factors, but it can be easily extended according
to the needs of each process. Inspecting the file  rwl_setup_param_weights.f helps in explaining what is
handled at the moment.

By running the pwhg_main program, you will get events with the appropriate <initrwgt> header, and with the
three weights added. The nominal weight is not included! You should add an appropriate line

<weight id='1'> No variations here, nominal weight </weight>

without any key=value pairs to also list the nominal weight.

ADDING WEIGHTS

By inserting the line

rwl_add 1

in the powheg.input file, a .lhe file is red in, and new weights are added to the previous ones. For
example, including

rwl_file '-'
rwl_add 1

<initrwgt>
<weightgroup name='PDFS'>
<weight id='4'> lhapdf=10803 </weight>
<weight id='5'> lhapdf=10804 </weight>
</weightgroup>
<weight id='6'> lhapdf=10802 facscfact=2 </weight>
</initrwgt>

three new weights will be added. Notice that in these way other
parameters may have different settings, according to the new
powheg.input file. Thus, by using this feature, the behaviour of the
older weight scheme can be fully reproduced.

Remember, however, that, as in the old weight scheme, you cannot
reweight in parameters that affect the generation of phase
space. Typically, the masses of final state particles or resonances
may do that.  If you need to do this, contact the authors of the
package.  Other subtle problems that may arise are, for example, if
the width of a resonance depends upon alpha_s, and thus on the PDF
set. This may cause inconsistencies. Always contact the authors of the
package if you find problems there.


EFFICIENCY ISSUES WITH MANY PDF SETS

If hundreds of weights are needed for pdf variations, POWHEG will have to load several PDF sets at once.
In order to avoid excessive memory paging when switching pdf's, POWHEG can handle events in groups.

By setting

rwl_group_events 1000

for example, POWHEG first computes one thousand events, then computes one weight variation for the
whole group, etc. In this way POWHEG has to keep 1000 events in memory, but this may reduce the
resident memory requirement for the pdf. One can play with this number to find optimal settings.

A flag is also available to limit the maximum number of pdf sets that the program can keep in memory:

lhapdf6maxsets 30

means that the program can keep up to 30 initialized pdf sets in memory. If yu have weights with more than 30
pdf's, when the program requests the 31st set, the set that was used less recently will be deallocated, and
a new set is allocated in his place. If this number exceeds the number of requested pdf's in the weights, all
needed pdf's will be kept in memory all time.

Thus, this number should be either higher than the number of pdf's requested in the weights, or very low (1,
or 2 if fullrwgt is on).