Esslinger, Hartmut (10 September 2013). "Keep It Simple". designmind.frogdesign.com/. Archived from the original on 1 February 2014. Retrieved 21 January 2014.
Esslinger, Hartmut (10 September 2013). "Keep It Simple". designmind.frogdesign.com/. Archived from the original on 1 February 2014. Retrieved 21 January 2014.