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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 parse_function(fn):
 """Get the source of a function and return its AST."""

  try:
    return parse_string(inspect.getsource(fn))
  except (IOError, OSError) as e:
    raise ValueError(
        'Cannot differentiate function: %s. Tangent must be able to access the '
        'source code of the function. Functions defined in a Python '
        'interpreter and functions backed by C extension modules do not '
        'have accessible source code.' % e)