Online Algebra Calculator
Many of users do not use powerful Maxima computer algebra system for systematic work, but for ad hoc algebraic calculations – equations, functions, matrixes, etc. only. To do this job, it is not useful to download the whole application from a web site.
-- Operator: not The 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.
There are also some inexact matches for not. Try ?? not to see them.
(%o1) true (%i2)