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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def findfile(self, old, new):
 """ return name of file to be patched or None """

    if exists(old):
      return old
    elif exists(new):
      return new
    else:
      # [w] Google Code generates broken patches with its online editor
      debug("broken patch from Google Code, stripping prefixes..")
      if old.startswith(b'a/') and new.startswith(b'b/'):
        old, new = old[2:], new[2:]
        debug("   %s" % old)
        debug("   %s" % new)
        if exists(old):
          return old
        elif exists(new):
          return new
      return None