While papers by others initially cited Schwinger, papers citing Feynman and employing Feynman diagrams appeared in 1950, and soon became prevalent. Students learned and used the powerful new tool that Feynman had created. Computer programs were later written to compute Feynman diagrams, providing a tool of unprecedented power. It is possible to write such programs because the Feynman diagrams constitute a formal language with a formal grammar. Marc Kac provided the formal proofs of the summation under history, showing that the parabolic partial differential equation can be re-expressed as a sum under different histories (that is, an expectation operator), what is now known as the Feynman–Kac formula, the use of which extends beyond physics to many applications of stochastic processes. To Schwinger, however, the Feynman diagram was "pedagogy, not physics".