code: def prefer(template_class,*extensions):
      ''' Makes a template class preferred for the given file extensions. If you
          don't provide any extensions, it will be preferred for all its already
          registered extensions:
        
          # Prefer Markdown for its registered file extensions:
          Lean.prefer(MarkdownTemplate)
          
          # Prefer Markdown only for the .md elxtensions:
          Lean.prefer(MarkdownTemplate, '.md')
      '''
  
      if len(extensions):
        for (ext,klasses) in Lean.template_mappings.items():
          if klasses.count(template_class):
            Lean.preferred_mappings[ext] = template_class
      else:
        for ext in extensions:
          ext = normalize(ext)
          Lean.register(template_class,ext)
          Lean.preferred_mappings[ext] = template_clasf prefer(template_class,*extensions):
      ''' Makes a template class preferred for the given file extensions. If you
          don't provide any extensions, it will be preferred for all its already
          registered extensions:
        
          # Prefer Markdown for its registered file extensions:
          Lean.prefer(MarkdownTemplate)
          
          # Prefer Markdown only for the .md elxtensions:
          Lean.prefer(MarkdownTemplate, '.md')
      '''
  
      if len(extensions):
        for (ext,klasses) in Lean.template_mappings.items():
          if klasses.count(template_class):
            Lean.preferred_mappings[ext] = template_class
      else:
        for ext in extensions:
          ext = normalize(ext)
          Lean.register(template_class,ext)
          Lean.preferred_mappings[ext] = template_class
docs: Makes a template class preferred for the given file extensions. If you
          don't provide any extensions, it will be preferred for all its already
          registered extensions:
        
          # Prefer Markdown for its registered file extensions:
          Lean.prefer(MarkdownTemplate)
          
          # Prefer Markdown only for the .md elxtensions:
          Lean.prefer(MarkdownTemplate, '.md')