The following cmavo is discussed in this section:
fu'a 
FUhA 
reverse Polish flag 
So far, the Lojban notational conventions have mapped fairly familiar kinds of mathematical discourse. The use of forethought operators may have seemed odd when applied to “+”, but when applied to “f” they appear as the usual functional notation. Now comes a sharp break. Reverse Polish (RP) notation represents something completely different; even mathematicians don't use it much. (The only common uses of RP, in fact, are in some kinds of calculators and in the implementation of some programming languages.)
In RP notation, the operator follows the operands. (Polish notation, where the operator precedes its operands, is another name for forethought mekso of the kind explained in Section 18.6.) The number of operands per operator is always fixed. No parentheses are required or permitted. In Lojban, RP notation is always explicitly marked by a fu'a at the beginning of the expression; there is no terminator. Here is a simple example:
The operands are re and ci; the operator is su'i.
Here is a more complex example:
li  fu'a  reboi  ci  pi'i  voboi  mu  pi'i  su'i 
thenumber  (RP!)  (two,  three,  times),  (four,  five,  times),  plus 
du  li  rexa 
equals  thenumber  twosix 
Here the operands of the first pi'i are re and ci; the operands of the second pi'i are vo and mu (with boi inserted where needed), and the operands of the su'i are reboi ci pi'i, or 6, and voboi mu pi'i, or 20. As you can see, it is easy to get lost in the world of reverse Polish notation; on the other hand, it is especially easy for a mechanical listener (who has a deep mental stack and doesn't get lost) to comprehend.
The operands of an RP operator can be any legal mekso operand, including parenthesized mekso that can contain any valid syntax, whether more RP or something more conventional.
In Lojban, RP operators are always parsed with exactly two operands. What about operators which require only one operand, or more than two operands? The null operand tu'o and the null operator ge'a provide a simple solution. A oneoperand operator like va'a always appears in a reverse Polish context as tu'o va'a. The tu'o provides the second operand, which is semantically ignored but grammatically necessary. Likewise, the threeoperand version of gei appears in reverse Polish as ge'a gei, where the ge'a effectively merges the 2nd and 3rd operands into a single operand. Here are some examples:
li  fu'a  ciboi  muboi  vu'u 
Thenumber  (RP!)  (three,  five,  minus) 
du  li  fu'a  reboi  tu'o  va'a  
equals  thenumber  (RP!)  two,  null,  negativeof.  
3 − 5 = 2

li  cinoki'oki'o  du 
Thenumber  30commacomma  equals 
li  fu'a  biboi  ciboi  panoboi  ge'a  gei  
thenumber  (RP!)  8,  (3,  10,  nullop),  exponentialnotation.  
30,000,000 = 3 × 10 ^ 8
