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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 Union(self, mask1, mask2):
 """Merges mask1 and mask2 into this FieldMask."""

    _CheckFieldMaskMessage(mask1)
    _CheckFieldMaskMessage(mask2)
    tree = _FieldMaskTree(mask1)
    tree.MergeFromFieldMask(mask2)
    tree.ToFieldMask(self)