code: def OE(element, value, transform=lambda x: x):
      
      return E(element, transform(value)) if value is not None else None
docs: Create an Optional Element.
  
      Returns an Element as ElementMaker would, unless value is None. Optionally the value can be
      transformed through a function.
  
      >>> OE('elem', None)
      None
  
      >>> lxml.etree.tostring(OE('elem', 'value'))
      <elem>value</elem>
  
      >>> lxml.etree.tostring(OE('elem', True, int))
      <elem>1</elem>