def flat_choice(when_broken, when_flat):
return FlatChoice(<EOL>validate_doc(when_broken),<EOL>validate_doc(when_flat)<EOL>)<EOL>
Gives the layout algorithm two options. ``when_flat`` Doc will be
    used when the document fit onto a single line, and ``when_broken`` is used
    when the Doc had to be broken into multiple lines.