Maxima Operator
notThe logical negation operator.
not is a prefix operator;
its operand is a Boolean expression, and its result is a Boolean value.
not forces evaluation (like is) of its operand.
The global flag prederror governs the behavior of not
when its operand cannot be determined to be true or false.
not prints an error message when prederror is true.
Otherwise, operands which do not evaluate to true or false are accepted,
and the result is a Boolean expression.