In reverse polish notation, how do you write a negative number?
For example, if you were to translate -3-5-8=x, would you write it as -3 5 - 8 - or -3 -5 + -8 +?
That's not about RPN, that's about how the language you use deals with the this bloody binary/unary '-' sign ambiguity.
In Forth for instance, the unary minus is a distinct operator (namely 'NEG').
Another possibility is to take advantage of the fact that the tokens of the language are whitespace delimited; in which case the compiler may accept negative literals. Such a language would let you write -3 -5 -8 + +, even though it defines '-' as a binary operator.