@StefanCK RPN notation was used in HP scientific calculators. This format does not require parenthesis and once you get used to it it is faster to make calculations (at least this is what HPers used to say). However the important aspect is that this expression format is super easy to parse. Pop operands from the stack, pop operator off the stack, compute result and push result back on stack. That's all. No complex grammar parsers required at all, no precendences / parenthesis.
So an expression like (2+3)*4 will be:
2 3 + 4 *
the rpn calculator will do:
• push 2 on stack
• push 3 on stack
• + operator: pop 2 and 3 from stack, compute addition (5) , push 5 on stack
• push 4 on stack
• * operator: pop 5 and 4 from stack, compute multiplication, push result (20) on stack.