Any sufficiently complete explanation of RPN contains half of a poorly written Forth implementation. ;-) Seriously, this is MUCH simpler than recursive descent

Any sufficiently complete explanation of RPN contains half of a poorly written Forth implementation. ;-) Seriously, this is MUCH simpler than recursive descent compilers or lex/yacc/bison. Add a good typing system to it, and you've got STOIC, a programming language from the 1970s.