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A contradiction: A AND NOT A

A AND NOT A is false on both rows of its table. The contradiction check holds, satisfiability is violated, and the simplifier reduces the formula to FALSE by the complement law.

A contradiction is a formula that is false on every row: no choice of inputs can make it true. A AND NOT A asks for A and its negation at once, which never happens. The engine's reading is deliberately literal, never is true exactly when A is true and A is false, and the two-row table shows the column false on both rows: the minterm list is empty and the maxterm list is ΠM(0, 1).

The contradiction check holds: never is false on every assignment. Satisfiability is violated, the formula can never be true, and the tautology check is violated with the counterexample A = FALSE. The simplifier applies the complement law, recorded as A AND NOT A = FALSE, taking two operators to the constant FALSE in one step. The CNF converter returns FALSE as one empty clause and the DNF converter as an empty disjunction, zero terms. The Karnaugh map has no cells to cover, draws no groups and derives FALSE, zero prime implicants. Gates for the original: 5 NAND and 2 NOR, NOR(NOR(A, A), A).

Contradictions are what you want from a safety property's negation: if “both greens on” is a contradiction, the lights are safe, which is what the traffic-light page checks. They are what you do not want from a set of rules: an access policy whose conditions contradict each other admits nobody, and a configuration with contradictory constraints has no valid state. The thing to watch is that an unsatisfiable rule rarely looks like A AND NOT A; it is usually spread across several clauses, and the satisfiability check is how it is found.

never := A AND NOT A · output

Exact checks

Always true (tautology)? No Counterexample: A = F · Exhaustively checked and solver cross-checked · 1 rows
Always false (contradiction)? Yes · Exhaustively checked and solver cross-checked · 2 rows
Can be true? No · Exhaustively checked and solver cross-checked · 2 rows

Truth table (2 rows)

A contradiction: A AND NOT A
#Anever
0FALSEFALSE
1TRUEFALSE

Plain-language reading

Beginnernever is true exactly when A is true and A is false.
In every other case, never is false.
About the operators:
- AND is true only when all of its parts are true.
- NOT reverses a value: true becomes false and false becomes true.
Intermediatenever is true exactly when A is true and A is false.
In every other case, never is false.
Advancednever is true exactly when A is true and A is false [conjunction ∧].
In every other case, never is false.
Formal notation: A ∧ ¬A

Logic diagram

A contradiction: A AND NOT A A contradiction: A AND NOT A A never AND NOT
A contradiction: A AND NOT A A contradiction: A AND NOT A A never AND NOT

Simplification, step by step

A AND NOT A
  1. Complement · A AND NOT A = FALSE
    A AND NOT A → FALSEResult: FALSE
FALSE · verified equivalent on every row · simplified, not minimal

Karnaugh map

A
0
1
0m0
0m1
FALSE · Derived FALSE. Exactly equivalent on every assignment.

Minterms and maxterms

never(A) = Σm()never(A) = ΠM(0, 1)

Variable order: A · 2 rows

Normal forms

Conjunctive normal form (CNF)

FALSE

1 clauses, 0 literals · complete · Complement ×1

Disjunctive normal form (DNF)

FALSE

0 terms, 0 literals · complete · Complement ×1

NAND-only and NOR-only

NAND-only form

NAND(NAND(A, NAND(A, A)), NAND(A, NAND(A, A)))

5 gates · verified equivalent on every row

NOR-only form

NOR(NOR(A, A), A)

2 gates · verified equivalent on every row

The same rule as code

Python
# -*- coding: utf-8 -*-
# Generated by Tangibool (tangibool.python/1). Do not edit by hand; regenerate from the source revision.
# Model: not stated
# Source revision: reference
# Interpretation status: not stated
"""Tangibool evaluator (decision model).

Generated deterministically from the validated Boolean model; it evaluates exactly that
model. It does not establish that the model is a complete, legal, or safe statement of a
real-world rule, and evaluating examples does not verify the rule.

Source revision: reference
Interpretation status: not stated
Exporter: 1.0.0 (format tangibool.python/1; engine 1.0.0; semantics tangibool-semantics/1)
Logic digest: 5542a051af1189095323c8274f7cba4b2a268d065449331a44bf652258eb6294
Assumptions digest: d58ee1ad38e23e5093e4baa1047b5ae70d6972e08f555e34ac3fa16df63c4a78

Inputs (dict keys, declared order) and generated parameter names:
    'A' -> input_A

Outputs (declared order) and their functions:
    'never' -> output_never(inputs)

Assumptions (the declared scope, checked by in_scope(inputs)):
    none; every complete assignment is in scope.

Input contract:
    Every required input must be exactly True or False (type(value) is bool). Missing
    inputs, None, strings, integers (including 0 and 1), and keys that are not declared
    inputs raise InputError before anything is evaluated, including inputs that a
    short-circuiting operator would never read. Results are always bool.

Scope:
    Decision functions raise OutOfScopeError when the inputs violate an assumption; an
    out-of-scope assignment is never turned into a False result. Pass check_scope=False
    to evaluate without the scope check, and use in_scope(inputs) to test it separately.

Usage:

    import tangibool_model as model

    model.evaluate({'A': False})
    # -> {'never': False}
    model.output_never({'A': False})
    # -> False
    model.evaluate({'A': 1})
    # raises InputError: 1 is an int, not a bool
"""

import collections.abc as _abc
from typing import Dict, Mapping, Tuple

__all__ = [
    "MODEL_INFO",
    "IDENTIFIER_MAP",
    "INPUT_NAMES",
    "OUTPUT_NAMES",
    "ASSUMPTIONS",
    "InputError",
    "OutOfScopeError",
    "minterm_index",
    "in_scope",
    "evaluate",
    "output_never",
]

MODEL_INFO = {
    "format": "tangibool.python/1",
    "mode": "decision",
    "model": None,
    "revision": "reference",
    "interpretation": None,
    "exporter": "1.0.0",
    "engine_version": "1.0.0",
    "semantics_version": "tangibool-semantics/1",
    "logic_digest": "5542a051af1189095323c8274f7cba4b2a268d065449331a44bf652258eb6294",
    "assumptions_digest": "d58ee1ad38e23e5093e4baa1047b5ae70d6972e08f555e34ac3fa16df63c4a78",
    "primary_output": "never",
    "unused_inputs": (),
    "subcircuits": (),
}

# Stable ID -> generated identifier (reversible mapping; names are exact, escaped strings).
IDENTIFIER_MAP = {
    "v_c164641b9b32": {"kind": "input", "name": "A", "identifier": "input_A"},
    "f_07e2a0a6c537": {"kind": "output", "name": "never", "identifier": "output_never"},
}

INPUT_NAMES = ("A",)
OUTPUT_NAMES = ("never",)
ASSUMPTIONS = ()

_DECLARED_INPUTS = frozenset(INPUT_NAMES)
_SCOPE_INPUTS = ()
_ASSUMPTION_FORMULAS = {entry["name"]: entry["formula"] for entry in ASSUMPTIONS}


class InputError(ValueError):
    """Raised when inputs are missing, undeclared, or not exactly True/False.

    Every required input must satisfy type(value) is bool. Missing inputs, None, strings,
    integers (including 0 and 1), and keys that are not declared inputs are rejected before
    anything is evaluated. A missing input is never treated as False. The individual problems
    are listed in the problems attribute.
    """

    def __init__(self, problems):
        self.problems = tuple(problems)
        super().__init__("Invalid inputs: " + "; ".join(self.problems))


class OutOfScopeError(ValueError):
    """Raised when the inputs violate the model's assumptions (its declared scope).

    Outside its assumptions the rule has no decision: this error is never a False result.
    The names of the violated assumptions are in the violated attribute.
    """

    def __init__(self, violated):
        self.violated = tuple(violated)
        details = ", ".join(name + " (" + _ASSUMPTION_FORMULAS.get(name, "?") + ")" for name in self.violated)
        super().__init__(
            "Out of scope: the inputs violate "
            + ("assumption " if len(self.violated) == 1 else "assumptions ")
            + details
            + ". The rule has no decision outside its assumptions; this is not a False result."
        )


def _short_repr(value: object) -> str:
    text = repr(value)
    return text if len(text) <= 60 else text[:57] + "..."


def _checked_inputs(inputs: Mapping[str, bool], required: Tuple[str, ...], check_scope: bool) -> Dict[str, bool]:
    """Validate inputs strictly before anything is evaluated and return a plain copy."""
    if type(check_scope) is not bool:
        raise TypeError("check_scope must be True or False")
    if not isinstance(inputs, _abc.Mapping):
        raise InputError(["inputs must be a mapping from input names to True/False, got " + type(inputs).__name__])
    problems = []
    for key in inputs:
        if type(key) is not str or key not in _DECLARED_INPUTS:
            declared = ", ".join(repr(name) for name in INPUT_NAMES) or "none"
            problems.append("undeclared input " + _short_repr(key) + " (declared inputs: " + declared + ")")
    needed = tuple(required) + (_SCOPE_INPUTS if check_scope else ())
    for name in needed:
        if name not in inputs:
            problems.append("missing required input " + repr(name))
    for key, value in inputs.items():
        if type(key) is str and key in _DECLARED_INPUTS and type(value) is not bool:
            problems.append(
                "input " + repr(key) + " must be True or False (type bool), got "
                + type(value).__name__ + " " + _short_repr(value)
            )
    if problems:
        raise InputError(list(dict.fromkeys(problems)))
    return dict(inputs.items())


def _require_bool_args(function: str, pairs: Tuple[Tuple[str, object], ...]) -> None:
    problems = [
        "argument " + repr(name) + " of " + function + " must be True or False (type bool), got "
        + type(value).__name__ + " " + _short_repr(value)
        for name, value in pairs
        if type(value) is not bool
    ]
    if problems:
        raise InputError(problems)


def minterm_index(*values: bool) -> int:
    """Row index of an assignment over a declared variable order.

    The first value is the most significant bit, so all-False is row 0. Used for indexed
    minterm (sum) and maxterm (product) forms.
    """
    index = 0
    for value in values:
        if type(value) is not bool:
            raise InputError(["minterm_index() takes True/False values, got " + type(value).__name__ + " " + _short_repr(value)])
        index = index * 2 + (1 if value else 0)
    return index


def _scope_values(values: Dict[str, bool]) -> Tuple[Tuple[str, bool], ...]:
    """Value of every enabled, confirmed assumption for validated inputs, in declared order."""
    return ()


def _require_in_scope(values: Dict[str, bool]) -> None:
    violated = tuple(name for name, holds in _scope_values(values) if not holds)
    if violated:
        raise OutOfScopeError(violated)


def in_scope(inputs: Mapping[str, bool]) -> bool:
    """True when the inputs satisfy every assumption (the model's declared scope).

    Requires the inputs the assumptions read; raises InputError for invalid inputs.
    """
    values = _checked_inputs(inputs, (), True)
    return all(holds for _, holds in _scope_values(values))


# ---- Formula bodies (inputs are validated by the public functions below) ----


def _compute_output_never(input_A: bool) -> bool:
    """Decision output 'never': A AND NOT A"""
    return input_A and (not input_A)


# ---- Public API ----


def output_never(inputs: Mapping[str, bool], *, check_scope: bool = True) -> bool:
    """Decision output 'never' (stable ID f_07e2a0a6c537).

    'never' := A AND NOT A

    Required inputs: 'A'.
    Raises InputError for missing, undeclared, or non-bool inputs, and OutOfScopeError
    when check_scope is True and the inputs violate an assumption.
    """
    values = _checked_inputs(inputs, ("A",), check_scope)
    if check_scope:
        _require_in_scope(values)
    output_never_value = _compute_output_never(values["A"])
    return output_never_value


def evaluate(inputs: Mapping[str, bool], *, check_scope: bool = True) -> Dict[str, bool]:
    """Evaluate every output (declared order); each output is computed once.

    Required inputs: 'A'.
    Raises InputError for missing, undeclared, or non-bool inputs, and OutOfScopeError
    when check_scope is True and the inputs violate an assumption.
    """
    values = _checked_inputs(inputs, ("A",), check_scope)
    if check_scope:
        _require_in_scope(values)
    output_never_value = _compute_output_never(values["A"])
    return {
        "never": output_never_value,
    }
PostgreSQL
-- Tangibool PostgreSQL export (tangibool.postgresql/1)
-- Target: PostgreSQL 16 or later with typed boolean inputs; tested with PostgreSQL 18.3 (PGlite 0.5.8).
-- Not a claim of compatibility with other SQL dialects.
-- Model: not stated
-- Source revision: reference
-- Interpretation status: not stated
-- Exporter: 1.0.0 (engine 1.0.0; semantics tangibool-semantics/1)
-- Logic digest: 5542a051af1189095323c8274f7cba4b2a268d065449331a44bf652258eb6294
-- Assumptions digest: d58ee1ad38e23e5093e4baa1047b5ae70d6972e08f555e34ac3fa16df63c4a78
-- Mode: decision
--
-- Read-only: this file contains only WITH / VALUES / SELECT statements. No DDL, DML,
-- procedures, credentials, network access, or database changes. The expressions are pure
-- Boolean expressions, so their result does not depend on SQL evaluation order.
--
-- NON-NULL INPUT PRECONDITION: NULL is not a truth value of this rule. Every fragment below
-- is defined only when every input column it reads is a non-NULL boolean. The guard template
-- (section 2) marks any NULL input as 'invalid_input' and any row that violates the
-- assumptions as 'out_of_scope', and fills output columns only for 'ok' rows; a NULL in an
-- output column of another row is an envelope marker, not a third truth value. Never wrap
-- the fragments in COALESCE(..., FALSE) or otherwise choose a meaning for unknown inputs.
-- This evaluates the supplied model; it does not show that the model is a complete, legal,
-- or safe statement of a real-world rule.
--
-- Input columns (stable ID: name -> column):
--   v_c164641b9b32: A -> "in_A"
-- Output columns:
--   f_07e2a0a6c537: never -> "out_never"
-- Assumptions (scope K; exported separately and never folded into an output):
--   none; every complete input row is in scope.

-- ----------------------------------------------------------------------------------------------
-- 1. Expression fragments (parenthesised; non-NULL boolean inputs required)
-- ----------------------------------------------------------------------------------------------
--
-- [out_never] output never (stable ID f_07e2a0a6c537)
--   never := A AND NOT A
--   PRECONDITION: "in_A" is a non-NULL boolean.
--   ("in_A" AND (NOT "in_A"))
--

-- ----------------------------------------------------------------------------------------------
-- 2. Guard template (read-only SELECT over an inputs list)
-- ----------------------------------------------------------------------------------------------
-- Replace the example rows between the markers with your own rows, or replace the inputs
-- CTE with a SELECT that provides the boolean columns listed in the header. status is
-- 'invalid_input' when any input is NULL, 'out_of_scope' when the assumptions are false, and
-- 'ok' otherwise; output columns are filled only for 'ok' rows (NULL elsewhere is an
-- envelope marker). Assumption columns are shown for rows with complete inputs.
WITH inputs ("row_id", "in_A") AS (
  -- tangibool:example-rows:begin
  VALUES
    (1, FALSE),
    (2, TRUE),
    (3, NULL::boolean)
  -- tangibool:example-rows:end
),
checked AS (
  SELECT
    inputs.*,
    CASE
      WHEN "in_A" IS NULL THEN 'invalid_input'
      ELSE 'ok'
    END AS "status"
  FROM inputs
)
SELECT
  "row_id",
  "in_A",
  "status",
  CASE WHEN "status" = 'ok' THEN ("in_A" AND (NOT "in_A")) END AS "out_never"
FROM checked
ORDER BY "row_id";
SMT-LIB
; Tangibool SMT-LIB export (tangibool.smtlib/1)
; Profile: SMT-LIB 2.6 Boolean subset (QF_UF): Bool constants, define-fun, let, push/pop, assert, check-sat, echo.
; Tested with Z3 5.1.0 (z3-solver 5.2.0, WebAssembly build).
; Model: not stated
; Source revision: reference
; Interpretation status: not stated
; Exporter: 1.0.0 (engine 1.0.0; semantics tangibool-semantics/1)
; Logic digest: 5542a051af1189095323c8274f7cba4b2a268d065449331a44bf652258eb6294
; Assumptions digest: d58ee1ad38e23e5093e4baa1047b5ae70d6972e08f555e34ac3fa16df63c4a78
; Mode: decision
;
; Reading results: each query block states what sat and unsat mean for its question. A sat
; answer comes with a witness or counterexample (ask for it with (get-model) after sat only);
; unsat settles that scoped query and is solver-checked, not an independently checked proof
; certificate; unknown settles nothing. Blocks are scoped with push/pop, so assertions never
; accumulate across questions. This checks the supplied model, not whether it is a complete,
; legal, or safe statement of a real-world rule.
;
; Symbols (stable ID: name -> symbol):
;   v_c164641b9b32: A -> in_A
;   f_07e2a0a6c537: never -> out_never

(set-option :produce-models true)
(set-info :smt-lib-version 2.6)
(set-logic QF_UF)

; ---- Inputs (declared order)
(declare-const in_A Bool) ; A

; ---- Outputs (dependencies first)
; output never (stable ID f_07e2a0a6c537): A AND NOT A
(define-fun out_never () Bool (and in_A (not in_A)))

; ---- Assumptions (the scope K; never folded into an output)
; K: conjunction of the enabled, confirmed assumptions (true when there are none)
(define-fun K () Bool true)

; ---- Definitions only: nothing is asserted and no check is run.
; Add (assert ...) and (check-sat) inside (push 1) ... (pop 1) to ask a question.
LaTeX
never = A \cdot \overline{A}

The common slip

Treating “unsatisfiable” as a bug in the checker when the rule is long. A rule with five conditions that reports no satisfying row is telling you two of its conditions exclude each other. The two-row example is the shape of that failure in miniature: A AND NOT A, and the complement law names it.

Questions

Why does the DNF have zero terms but the CNF has one?

FALSE as a disjunction is the empty one, no term to make it true; FALSE as a conjunction is a single clause with no literals, which cannot be satisfied. The converters report the term and literal counts that make each form read as the constant, and the table confirms both.

Related

Laws

Operators and gates

Tools

Practice

See also

Worked examples

Generated from the Tangibool engine, semantics tangibool-semantics/1, engine 1.0.0, build . Every table, map, simplification and verdict on these pages was computed by the Tangibool engine when the site was built, and every claim was re-checked by the same exact checker the app uses. Limits are stated where they apply. © 2026 Foreranger LTD.