Reptiles is a lithograph print by the Dutch artist M. C. Escher first printed in March 1943. It depicts a desk upon which is a 2D drawing of a tessellated pattern of reptiles and hexagons. The reptiles at one edge of the drawing emerge into three dimensional reality, come to life and appear to crawl over a series of symbolic objects (a book on nature, a geometers triangle, a three dimensional/pentagonal dodecahedron, a pewter bowl containing a box of matches and a box of cigarettes) to eventually re-enter the drawing at its opposite edge. Other objects on the desk that may also be symbolic are...

P′′ is a primitive computer programming language created by Corrado Böhm in 1964 to describe a family of Turing machines. == Definition == P ′ ′ {\displaystyle {\mathcal {P}}^{\prime \prime }} (hereafter written P′′) is formally defined as a set of words on the four-instruction alphabet {R, λ, (, )}, as follows: === Syntax === R and λ are words in P′′. If p and q are words in P′′, then pq is a word in P′′. If q is a word in P′′, then (q)...