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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 iter_fields(node):
 """Iterate over all fields of a node, only yielding existing fields."""

    # CPython 2.5 compat
    if not hasattr(node, '_fields') or not node._fields:
        return
    for field in node._fields:
        try:
            yield field, getattr(node, field)
        except AttributeError:
            pass