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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 _get_anchor(module_to_name, fullname):
 """Turn a full member name into an anchor. Args: module_to_name: Dictionary mapping modules to short names. fullname: Fully qualified name of symbol. Returns: HTML anchor string. The longest module name prefix of fullname is removed to make the anchor. Raises: ValueError: If fullname uses characters invalid in an anchor. """

  if not _anchor_re.match(fullname):
    raise ValueError("'%s' is not a valid anchor" % fullname)
  anchor = fullname
  for module_name in module_to_name.values():
    if fullname.startswith(module_name + "."):
      rest = fullname[len(module_name)+1:]
      # Use this prefix iff it is longer than any found before
      if len(anchor) > len(rest):
        anchor = rest
  return anchor