 After finishing the Z1 in 1938, Zuse discovered that the calculus he had independently devised already existed and was known as propositional calculus.What Zuse had in mind, however, needed to be much more powerful (propositoinal calculus is not Turing-complete and is not able to describe even simple arithmetic calculations).In May 1939 he described his plans for the development of what would become Plankalkül.