code: def _SConstruct_exists(dirname='', repositories=[], filelist=None):
      
      if not filelist:
          filelist = ['SConstruct', 'Sconstruct', 'sconstruct']
      for file in filelist:
          sfile = os.path.join(dirname, file)
          if os.path.isfile(sfile):
              return sfile
          if not os.path.isabs(sfile):
              for rep in repositories:
                  if os.path.isfile(os.path.join(rep, sfile)):
                      return sfile
      return None
docs: This function checks that an SConstruct file exists in a directory.
      If so, it returns the path of the file. By default, it checks the
      current directory.