In the imperative
3 + 4 =>the symbol "+" acts as the name of a built-in procedure to be applied to the numbers 3 and 4. The symbol "+" is called an `infix operator' because you can run the procedure by writing the name between the inputs, although the following "parenthesised prefix" syntax would work just as well:
+(3, 4) =>
** 7
Just as "+" can be used for addition, so can the asterisk "*" be used as
an infix operator for multiplication. So:
3 * 5 =>
** 15
999 * 999 =>
** 998001
Pop-11 provides many built in procedures whose names function as infix
operators. Other examples are the equality predicates "=" (which tests
for the same type of object with equal components), "==" (which tests
for the very same object), the STRING concatenator "><", the generic
structure concatenator "<>", the list constructor "::", and other
arithmetic operators for division "/", remainder "rem", subtraction "-"
and exponentiation "**". (That is not a complete list.)
Infix operators have a numerical precedence which determines how they should be grouped if combined in complex expressions, like "x + y * z", and this is explained in Chapter 4. Chapter 4 also shows how users can define their own infix operators.