188.8.131.52. Bit-wise and logical NOT (NOT)
This unary operator calculates the logical negate of the operand. The calculation is done on integer numbers, thus the operand is converted to integer value. The operator inverts each bit of the operand. If the operand is undef the result is -1, which is the internal value for TRUE.
Great care should be taken when using the NOT operator in logical expressions. The precedence of the operator is higher than that of any binary operator. Therefore the expression
not true or false
is true, while
not (true or true)