def compute_content(self, layout):
<EOL>out = self.out<EOL>try:<EOL><INDENT>for child in layout.children:<EOL><INDENT>stream = StringIO()<EOL>self.out = stream<EOL>child.accept(self)<EOL>yield stream.getvalue()<EOL><DEDENT><DEDENT>finally:<EOL><INDENT>self.out = out<EOL><DEDENT>
trick to compute the formatting of children layout before actually
        writing it

        return an iterator on strings (one for each child element)