def unique(iterables):
included_elements = set()<EOL>def included(element):<EOL><INDENT>result = element in included_elements<EOL>included_elements.add(element)<EOL>return result<EOL><DEDENT>return [element for elements in iterables for element in elements<EOL>if not included(element)]<EOL>
Create an iterable from the iterables that contains each element once.

    :return: an iterable over the iterables. Each element of the result
      appeared only once in the result. They are ordered by the first
      occurrence in the iterables.