"Filled with with urban detritus, the vitrine memorializes the remains of a 1972 action performed by Beuys in Berlin following the left-wing May Day demonstration; he and two students used a red broom to sweep up the rubbish in Karl-Marx-Platz." (Barron, 2009)
In 1960s Joseph Beuys satirised the Wall by suggesting that it should be raised by 5cm "for better proportion" [Tate Modern, 2011].
Barron, Stephanie. (2009) "Blurred Boundaries: The Art of Two Germanys between Myth and History" in Art of Two Germanys: Cold War Cultures, pp.12-33.
Curiger, B. & Whiteread, R. (2010) The Process of Drawing is Like Writing a Diary" [Online]Tate Modern  Track 6: Lightning with Stag in its Glare, Beuys and the Berlin Wall. [Online] http://www.tate.org.uk/modern/exhibitions/beuys/mp3/06_berlin_wall.mp3