[your code here] Pure RPN calculator

[Max Haughton via Digitalmars-d]

Semi-Functional/pure RPN calculator: 
https://run.dlang.io/is/JGkBZx

This is probably too long, but it demonstrates the compiler 
enforced safety and purity (State is passed through the fold), 
while also showing off the higher level parts of Phobos (Use of 
fold).