# Boolean operators

The simplest operator which has an operand of mode `BOOL` is `NOT`. If its operand is `TRUE`, it yields `FALSE`. Conversely, if its operand is `FALSE`, it yields `TRUE`. The operator `ODD` yields `TRUE` if its operand is an odd integer and `FALSE` if it is even. The operators can be combined, so

       NOT ODD 2

yields `TRUE`.

`ABS` converts its operand of mode `BOOL` and yields an integer: `ABS TRUE` yields `1`, `ABS FALSE` yields `0`.

Boolean dyadic operators come in two kinds: those that take operands of mode `BOOL`, yielding `TRUE` or `FALSE`, and those that operate on operands of other modes.

Two dyadic operators are declared in the standard prelude which take operands of mode `BOOL`. The operator `AND` (alternative representation `&`) yields `TRUE` if, and only if, both its operands yield `TRUE`, so that

       t AND f

yields `FALSE` (`t` and `f` were declared earlier). Both the operands are elaborated before the operator (but see the section later on pseudo-operators). The priority of `AND` is 3.

The operator `OR` yields `TRUE` if at least one of its operands yields `TRUE`. Thus

       t OR f

yields `TRUE`. It has no alternative representation. Again, both operands are elaborated before the operator. The priority of `OR` is 2.

You will learn in chapter 6 how to define new operators if you need them.