diff --git a/Slides/presentation_helper.py b/Slides/presentation_helper.py
index 6683708..d63970f 100644
--- a/Slides/presentation_helper.py
+++ b/Slides/presentation_helper.py
@@ -1,309 +1,311 @@
 #!/usr/bin/env pyton
 ################################################################
 
 from sympy import *
 import base64
 import sympy
 import PyPDF2
 import io
 import IPython
 from IPython.display import display
 from IPython.display import HTML
 import os
 import fnmatch
 import shutil
 import subprocess
 import matplotlib.pyplot as plt
 import matplotlib.figure
 from IPython.core.interactiveshell import InteractiveShell
 from wand.image import Image as WImage
 from wand.color import Color
 from io import BytesIO
 from tempfile import NamedTemporaryFile
 from tempfile import mkdtemp
 ################################################################
 
 
 def findImageFile(filename):
     if os.path.isfile(filename):
         return filename
 
     tool_dir = os.path.dirname(__file__)
     image_dir = os.path.join(tool_dir, '..', 'images')
     pattern = os.path.basename(filename)
     found = []
     for root, dirnames, filenames in os.walk(image_dir):
         for filename in fnmatch.filter(filenames, pattern):
             found.append(os.path.join(root, filename))
 
     if len(found) == 0:
         raise Exception("file not found: {0}".format(filename))
     if len(found) == 1:
         return found[0]
 
     raise Exception("Several files were found:\n{0}".format(found))
 
 
 ################################################################
 def _print_latex_png(o):
     o = o._repr_latex_()
     dvioptions = None
     exprbuffer = BytesIO()
     # from IPython.core.debugger import Tracer
     # Tracer()()
     preview(o, output='png', viewer='BytesIO',
             outputbuffer=exprbuffer, packages=('xcolor',),
             dvioptions=dvioptions)
     return exprbuffer.getvalue()
 
 ################################################################
 
 
 def registerFormatter(_type, _format, formatter_functor):
     f = InteractiveShell.instance().display_formatter.formatters[_format]
     f.for_type(_type, func=formatter_functor)
 
 ################################################################
 
 
 def init_printing():
     sympy.init_printing()
     registerFormatter(IPython.core.display.Math, 'image/png', _print_latex_png)
     # f = InteractiveShell.instance().display_formatter.formatters['image/png']
     # f.for_type(IPython.core.display.Math, func=_print_latex_png)
 
 ################################################################
 
 
 VIDEO_TAG = """<video controls>
  <source src="data:video/x-m4v;base64,{0}" type="video/mp4">
  Your browser does not support the video tag.
 </video>"""
 
 
 def anim_to_html(anim):
     if not hasattr(anim, '_encoded_video'):
         with NamedTemporaryFile(suffix='.mp4') as f:
             anim.save(f.name, fps=1./(anim._interval*1e-3),
                       extra_args=['-vcodec', 'libx264',
                                   '-pix_fmt', 'yuv420p'])
             video = open(f.name, "rb").read()
         toto = base64.b64encode(video).decode()
         # print anim._encoded_video
 
     return VIDEO_TAG.format(toto)
 
 ################################################################
 
 
 def display_animation(anim):
     plt.close(anim._fig)
     return HTML(anim_to_html(anim))
 
 ################################################################
 
 
 def display_images(objs, display_flag=True, **kwargs):
     htmls = [display_image(o, display_flag=False, **kwargs)
              for o in objs]
 
     width = 100/len(htmls)
 
     htmls_withdiv = ["" for h in htmls]
     for i, h in enumerate(htmls):
         htmls_withdiv[i] = '<div style="width:{0}%;'.format(width)
         if i == 0:
             htmls_withdiv[i] += 'float:left'
         if i == len(htmls) - 1:
             htmls_withdiv[i] += 'float:right'
         htmls_withdiv[i] += '"> {0} </div>'.format(h)
     html = '\n'.join(htmls_withdiv)
     html = '<div>' + html + '</div>'
     if display_flag:
         display(HTML(html))
     return html
 
 
 def display_image(obj, resolution=150, width=None, height=None,
                   display_flag=True):
 
     if type(obj) == matplotlib.figure.Figure:
         png = InteractiveShell.instance().display_formatter.format(obj)
 
     elif type(obj) == str:
         try:
             fname = findImageFile(obj)
 
         except Exception as e:
             return str(e)
 
         img = WImage(filename=fname, resolution=resolution)
         img.trim()
         with Color('#FDFDFD') as white:
             twenty_percent = int(65535 * 0.2)
             img.transparent_color(white, alpha=0.0, fuzz=twenty_percent)
 
         if (width is None) and (height is None):
             display(img)
             return img
 
         png = InteractiveShell.instance().display_formatter.format(img)
     else:
         raise Exception('unknown obj type for an image')
 
     size_tag = ""
     if width is not None:
         size_tag += 'width="{0}"'.format(width)
     if height is not None:
         size_tag += 'height="{0}"'.format(height)
 
     data = png[0]['image/png']
     data = base64.b64encode(data).decode('utf-8')
     html = r'<img alt="You cannot see the image with this browser/mailer" '
     r'src="data:image/png;base64,{0}"'.format(data)
 
     if size_tag != '':
         html += '{0}'.format(size_tag)
     html += '/>'
 
     # print(html)
     if display_flag:
         display(HTML(html))
     return html
 
 ################################################################
 
 
 def display_graph(obj, **kwargs):
     if not isinstance(obj, str):
         raise RuntimeError(
             'Graphs should be passed as a str object storing the graph'
             ' in dot format')
 
     with NamedTemporaryFile(suffix='.svg') as f:
         p = subprocess.Popen('dot -Tsvg -o' + f.name, shell=True,
                              stdin=subprocess.PIPE)
         p.communicate(input=obj.encode())
         display_image(f.name, **kwargs)
 
 ################################################################
 
 
 def makeAnimationFromImageFiles(filenames, framerate):
     temp_dir = mkdtemp()
     ext = None
     for idx, f in enumerate(filenames):
         _dir = os.path.dirname(f)
         base = os.path.basename(f)
         base, _ext = os.path.splitext(base)
         ext = _ext
         if ext == '.pdf':
             _png = os.path.join(_dir, base+".png")
             subprocess.call('convert {0} {1}'.format(f, _png), shell=True)
             ext = ".png"
             f = _png
         new_name = "file{0:05d}{1}".format(idx, ext)
         new_name = os.path.join(temp_dir, new_name)
         # print base, ext
         # print new_name
         # print "copy {0} to {1}".format(f,new_name)
         shutil.copyfile(f, new_name)
 
     pattern = os.path.join(temp_dir, 'file%05d'+ext)
     with NamedTemporaryFile(suffix='.mp4') as f:
         cmd = ('ffmpeg -y -framerate {2} -i {0} -vf "scale=trunc(iw/2)*2:'
                'trunc(ih/2)*2" -c:v libx264 -r 30 '
                '-pix_fmt yuv420p {1}').format(pattern, f.name, framerate)
         # print cmd
         subprocess.call(cmd, shell=True)
         video = open(f.name, "rb").read()
     toto = video.encode("base64")
 
     shutil.rmtree(temp_dir)
 
     return HTML(VIDEO_TAG.format(toto))
 
 
 ################################################################
 
 
 def displayClassDiagram(classes, resolution=150,
                         width=None, height=None):
 
     png = makeClassDiagramImage(classes, resolution,
                                 width, height)
     img = WImage(filename=png, resolution=resolution)
     with Color('#FDFDFD') as white:
         twenty_percent = int(65535 * 0.2)
         img.transparent_color(white, alpha=0.0, fuzz=twenty_percent)
 
     png = InteractiveShell.instance().display_formatter.format(img)
 
     size_tag = ""
     if width is not None:
         size_tag += 'width="{0}"'.format(width)
     if height is not None:
         size_tag += 'height="{0}"'.format(height)
 
     data = png[0]['image/png']
-    html = r'<img alt="You cannot see the image with this browser/mailer" '
-    'src="data:image/png;base64,{0}" {1}/>'.format(
-        base64.b64encode(data).decode('utf8'), size_tag)
+    html = (r'<img alt="You cannot see the image with this browser/mailer" '
+            + 'src="data:image/png;base64,{0}" {1}/>'.format(
+                base64.b64encode(data).decode('utf8'), size_tag))
     display(HTML(html))
     return html
 
 ################################################################
 
 
 def makeClassDiagramImage(classes, resolution=150,
                           width=None, height=None):
 
     with NamedTemporaryFile(suffix='.classes', delete=False) as f:
         f.write(classes.encode('utf-8'))
         f.flush()
 
-    fout = os.path.splitext(f.name)[0] + '.png'
+    fout = os.path.splitext(f.name)[0] + '.svg'
     _command = ('class_dumper_dot.py --collaboration_no '
                 '-c {0} -o {1} -f svg'.format(f.name, fout))
     # print(_command)
-    subprocess.call(_command, shell=True)
+    ret = subprocess.call(_command, shell=True)
+    if ret:
+        raise RuntimeError('could not launch commad:', _command)
     return fout
 
 ################################################################
 
 
 def pdf_page_to_png(src_filename, pagenums=None, resolution=72):
     """
     Returns specified PDF page as wand.image.Image png.
     :param PyPDF2.PdfFileReader src_pdf: PDF from which to take pages.
     :param int pagenum: Page number to take.
     :param int resolution: Resolution for resulting png in DPI.
     """
 
     src_pdf = PyPDF2.PdfFileReader(file(src_filename, "rb"))
     # print dir(src_pdf)
 
     # print src_pdf.numPages
 
     if pagenums is None:
         pagenums = [0]
 
     res = []
     for n in pagenums:
         dst_pdf = PyPDF2.PdfFileWriter()
         dst_pdf.addPage(src_pdf.getPage(n))
 
         pdf_bytes = io.BytesIO()
         dst_pdf.write(pdf_bytes)
         pdf_bytes.seek(0)
 
         img = WImage(file=pdf_bytes, resolution=resolution)
         img.convert("png")
         res.append(img)
 
     return res
 
 ################################################################
 
 
 init_printing()