FUhA (http://www.lojban.org/en/publications/reference_grammar/chapter18.html)

Indicates that the following mathematical expression is to be interpreted as
reverse Polish (RP), a mode in which mathematical operators follow their operands.

li fu'a reboi re[boi] su'i du li vo the-number [RP!] two, two, plus equals
the-number four 2 + 2 = 4

