The beauty of the prefix / prefix means you don't need parentheses. Brackets are used in infix notation because ambiguities exist, for example:
a + b - c
This may mean one of the following:
(a + b) - c
a + (b - c)
However, in postfix notation, they clearly differ:
a b + c -
a b c - +
No parentheses / brackets are needed to ensure the order of evaluation in postfix notation.
see also
Convert from infix
infix , . " " Edsger Dijkstra.
.