RPN. In 2013, the HP Prime introduced a 128-level form of entry RPN called advanced RPN. By late 2017, only the 12C, 12C Platinum, 17bii+, 35s and Prime remain active HP models supporting reverse Polish notation. WP 31S and WP 34S The community-developed calculators WP 31S and WP 34S, which are based on the HP 20b/HP 30b hardware platform, support Hewlett-Packard-style classical reverse Polish notation with either a four- or an eight-level stack. A seven-level stack had been implemented in the MITS 7400C scientific desktop calculator in 1972 and an eight-level stack was already suggested by John A. Ball in 1978. Sinclair Radionics In Britain, Clive Sinclair's Sinclair Scientific and Scientific Programmable models used reverse Polish notation. Commodore In 1974, Commodore produced the Minuteman *6 (MM6) without key and the Minuteman *6X (MM6X) with key, both implementing a form of two-level RPN. The SR4921 RPN came with a variant of four-level RPN with stack levels named X, Y, Z, and W (rather than T) and an key (for "entry"). In contrast to Hewlett-Packard's reverse Polish notation implementation, W filled with 0 instead of its contents being duplicated on stack drops. Prinztronic Prinz and Prinztronic were own-brand trade names of the British Dixons photographic and electronic goods stores retail chain, later rebranded as Currys Digital stores, and became part of DSG International. A variety of calculator models was sold in the 1970s under the Prinztronic brand, all made for them by other companies. Among these was the PROGRAM Programmable Scientific Calculator which featured reverse Polish notation. Heathkit The Aircraft Navigation Computer Heathkit OC-1401/OCW-1401 used five-level RPN in 1978. Soviet Union Soviet programmable calculators (MK-52, MK-61, B3-34 and earlier B3-21 models) used reverse Polish notation for both automatic mode and programming. Modern Russian calculators MK-161 and MK-152, designed and manufactured in Novosibirsk since 2007 and offered by Semico, are backwards compatible with them. Their extended architecture is also based on reverse Polish notation. Other Existing implementations using reverse Polish notation include: Stack-oriented programming languages such as: Forth STOIC Factor PostScript page description language BibTeX Befunge Joy IPTSCRAE Lotus 1-2-3 and Lotus Symphony formulas RPL (aka Reverse Polish Language), a programming language for the Commodore PET around 1979/1981 RPL (aka Reverse Polish Lisp), a programming language for Hewlett-Packard calculators between 1984 and 2015 RPNL (Reverse Polish Notation Language) Hardware calculators: Some Hewlett-Packard science/engineering and business/finance calculators Semico calculators SwissMicros calculators Some APF calculators