Citations:Penrose stairs

possibly a reference to Penrose's specific stairs, but probably generic

 * 2012, Christopher Davis, Pieces of Thought ISBN 147971755X, page 34:
 * The world just escalated, climbing up the Penrose stairs motorized and unmotivated we've populated this big blue sphere with war
 * The world just escalated, climbing up the Penrose stairs motorized and unmotivated we've populated this big blue sphere with war