I discovered my old HP-35 calculator, a truly revolutionary calculator when I bought it for $395 in 1973, because it was shirt-pocket sized but could do complex calculations, trig, log, and exponentiation, and others over 200 orders of magnitude at a time the state of the art was four-function calculators that were 50% fatter. The HP-35 was also revolutionary in its use of Reverse Polish Notation to enable the solution of complex equations, a logical way to think but very new for the time, and it fell to this user's manual (36 small pages plus fold-out at the front) to explain how this worked, and to give many illustrated examples and carefully step through their solutions. An HP-42s simulator, a later calculator but very similar and also reverse Polish, is available as an app for (at least) iOS.