def OE(element, value, transform=lambda x: x):
return E(element, transform(value)) if value is not None else None<EOL>
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>