At most one of three: mutual exclusion
Mutual exclusion, NOT ((A AND B) OR (A AND C) OR (B AND C)): at most one of three inputs may be true. The page shows how De Morgan turns the negated majority into three pairwise clauses, with the map and gate counts.
The formula is the negation of majority: it is true when no pair of inputs is true together, so when zero or one input is true. The engine's reading says it the same way: at_most_one holds exactly when none of A AND B, A AND C, B AND C holds. On the eight-row table the output is true on four rows, Σm(0, 1, 2, 4): the all-false row and the three rows with a single true input. Unlike exactly-one, the all-false row is allowed.
The tautology check is violated with the counterexample A = FALSE, B = TRUE, C = TRUE, and satisfiability holds with the witness A = FALSE, B = FALSE, C = FALSE. The simplifier found no law that shortens the six-operator expression. The CNF converter applies De Morgan once and produces (NOT A OR NOT B) AND (NOT A OR NOT C) AND (NOT B OR NOT C), three clauses of two literals. The DNF has three two-literal terms, the same three groups the Karnaugh map draws, three prime implicants. Both the NAND-only and the NOR-only forms need 19 gates.
At-most-one is the safety half of a selection rule: no two motors driving the same shaft, no two payment methods charged, no two writers holding the same lock. The CNF reads directly as the list of forbidden pairs, which is how a linter or a configuration validator would state it. The thing to watch is the other half: at-most-one says nothing about at-least-one, and a system where nothing is selected satisfies it. Combine it with the exactly-one page when an empty selection is also an error.
at_most_one := NOT ((A AND B) OR (A AND C) OR (B AND C)) · output
Exact checks
Truth table (8 rows)
| # | A | B | C | at_most_one |
|---|---|---|---|---|
| 0 | FALSE | FALSE | FALSE | TRUE |
| 1 | FALSE | FALSE | TRUE | TRUE |
| 2 | FALSE | TRUE | FALSE | TRUE |
| 3 | FALSE | TRUE | TRUE | FALSE |
| 4 | TRUE | FALSE | FALSE | TRUE |
| 5 | TRUE | FALSE | TRUE | FALSE |
| 6 | TRUE | TRUE | FALSE | FALSE |
| 7 | TRUE | TRUE | TRUE | FALSE |
Plain-language reading
Beginnerat_most_one is true exactly when the following condition does not hold:
1. at least one of the following holds (several may hold):
1.1. both A and B are true
1.2. both A and C are true
1.3. both B and C are true
In every other case, at_most_one is false.
About the operators:
- NOT reverses a value: true becomes false and false becomes true.
- OR is inclusive: it is true when at least one part is true, including when several are.
- AND is true only when all of its parts are true.
Intermediateat_most_one is true exactly when the following condition does not hold:
1. at least one of the following holds (several may hold):
1.1. both A and B are true
1.2. both A and C are true
1.3. both B and C are true
In every other case, at_most_one is false.
Advancedat_most_one is true exactly when the following condition does not hold [negation ¬]:
1. at least one of the following holds (several may hold) [inclusive disjunction ∨]:
1.1. both A and B are true [conjunction ∧]
1.2. both A and C are true [conjunction ∧]
1.3. both B and C are true [conjunction ∧]
In every other case, at_most_one is false.
Formal notation: ¬((A ∧ B) ∨ (A ∧ C) ∨ (B ∧ C))
Logic diagram
Simplification, step by step
The engine found no law that shortens this expression; it is already as short as its rewriting laws can make it.
NOT ((A AND B) OR (A AND C) OR (B AND C))
Karnaugh map
- cells m0, m1
NOT A AND NOT B - cells m0, m2
NOT A AND NOT C - cells m0, m4
NOT B AND NOT C
Minterms and maxterms
at_most_one(A, B, C) = Σm(0, 1, 2, 4)at_most_one(A, B, C) = ΠM(3, 5, 6, 7)Variable order: A, B, C · 8 rows
Normal forms
Conjunctive normal form (CNF)
(NOT A OR NOT B) AND (NOT A OR NOT C) AND (NOT B OR NOT C)3 clauses, 6 literals · complete · De Morgan's law ×1
Disjunctive normal form (DNF)
(NOT A AND NOT B) OR (NOT A AND NOT C) OR (NOT B AND NOT C)3 terms, 6 literals · complete · De Morgan's law ×1, Distribution ×2, Absorption ×2, Idempotence ×1
NAND-only and NOR-only
NAND-only form
NAND(NAND(NAND(NAND(NAND(A, B), NAND(A, C)), NAND(NAND(A, B), NAND(A, C))), NAND(B, C)), NAND(NAND(NAND(NAND(A, B), NAND(A, C)), NAND(NAND(A, B), NAND(A, C))), NAND(B, C)))19 gates · verified equivalent on every row
NOR-only form
NOR(NOR(NOR(NOR(NOR(A, A), NOR(B, B)), NOR(NOR(A, A), NOR(C, C))), NOR(NOR(NOR(A, A), NOR(B, B)), NOR(NOR(A, A), NOR(C, C)))), NOR(NOR(B, B), NOR(C, C)))19 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: 4ef3ccb1fa7e9a22f2faf9222cfc7bf8399a90e1255462fca845fba40d234c5a
Assumptions digest: d58ee1ad38e23e5093e4baa1047b5ae70d6972e08f555e34ac3fa16df63c4a78
Inputs (dict keys, declared order) and generated parameter names:
'A' -> input_A
'B' -> input_B
'C' -> input_C
Outputs (declared order) and their functions:
'at_most_one' -> output_at_most_one(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, 'B': False, 'C': False})
# -> {'at_most_one': True}
model.output_at_most_one({'A': False, 'B': False, 'C': False})
# -> True
model.evaluate({'A': 1, 'B': False, 'C': False})
# 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_at_most_one",
]
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": "4ef3ccb1fa7e9a22f2faf9222cfc7bf8399a90e1255462fca845fba40d234c5a",
"assumptions_digest": "d58ee1ad38e23e5093e4baa1047b5ae70d6972e08f555e34ac3fa16df63c4a78",
"primary_output": "at_most_one",
"unused_inputs": (),
"subcircuits": (),
}
# Stable ID -> generated identifier (reversible mapping; names are exact, escaped strings).
IDENTIFIER_MAP = {
"v_f70ccd60a05f": {"kind": "input", "name": "A", "identifier": "input_A"},
"v_b3b35c3e81d9": {"kind": "input", "name": "B", "identifier": "input_B"},
"v_5c5313465744": {"kind": "input", "name": "C", "identifier": "input_C"},
"f_b08deefb996d": {"kind": "output", "name": "at_most_one", "identifier": "output_at_most_one"},
}
INPUT_NAMES = ("A", "B", "C")
OUTPUT_NAMES = ("at_most_one",)
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_at_most_one(input_A: bool, input_B: bool, input_C: bool) -> bool:
"""Decision output 'at_most_one': NOT ((A AND B) OR (A AND C) OR (B AND C))"""
return not ((input_A and input_B) or (input_A and input_C) or (input_B and input_C))
# ---- Public API ----
def output_at_most_one(inputs: Mapping[str, bool], *, check_scope: bool = True) -> bool:
"""Decision output 'at_most_one' (stable ID f_b08deefb996d).
'at_most_one' := NOT ((A AND B) OR (A AND C) OR (B AND C))
Required inputs: 'A', 'B', 'C'.
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", "B", "C"), check_scope)
if check_scope:
_require_in_scope(values)
output_at_most_one_value = _compute_output_at_most_one(values["A"], values["B"], values["C"])
return output_at_most_one_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', 'B', 'C'.
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", "B", "C"), check_scope)
if check_scope:
_require_in_scope(values)
output_at_most_one_value = _compute_output_at_most_one(values["A"], values["B"], values["C"])
return {
"at_most_one": output_at_most_one_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: 4ef3ccb1fa7e9a22f2faf9222cfc7bf8399a90e1255462fca845fba40d234c5a
-- 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_f70ccd60a05f: A -> "in_A"
-- v_b3b35c3e81d9: B -> "in_B"
-- v_5c5313465744: C -> "in_C"
-- Output columns:
-- f_b08deefb996d: at_most_one -> "out_at_most_one"
-- 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_at_most_one] output at_most_one (stable ID f_b08deefb996d)
-- at_most_one := NOT ((A AND B) OR (A AND C) OR (B AND C))
-- PRECONDITION: "in_A", "in_B", "in_C" are non-NULL booleans.
-- (NOT (("in_A" AND "in_B") OR ("in_A" AND "in_C") OR ("in_B" AND "in_C")))
--
-- ----------------------------------------------------------------------------------------------
-- 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", "in_B", "in_C") AS (
-- tangibool:example-rows:begin
VALUES
(1, FALSE, FALSE, FALSE),
(2, FALSE, FALSE, TRUE),
(3, FALSE, TRUE, FALSE),
(4, FALSE, TRUE, TRUE),
(5, TRUE, FALSE, FALSE),
(6, TRUE, FALSE, TRUE),
(7, TRUE, TRUE, FALSE),
(8, TRUE, TRUE, TRUE),
(9, NULL::boolean, FALSE, FALSE)
-- tangibool:example-rows:end
),
checked AS (
SELECT
inputs.*,
CASE
WHEN "in_A" IS NULL OR "in_B" IS NULL OR "in_C" IS NULL THEN 'invalid_input'
ELSE 'ok'
END AS "status"
FROM inputs
)
SELECT
"row_id",
"in_A",
"in_B",
"in_C",
"status",
CASE WHEN "status" = 'ok' THEN (NOT (("in_A" AND "in_B") OR ("in_A" AND "in_C") OR ("in_B" AND "in_C"))) END AS "out_at_most_one"
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: 4ef3ccb1fa7e9a22f2faf9222cfc7bf8399a90e1255462fca845fba40d234c5a ; 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_f70ccd60a05f: A -> in_A ; v_b3b35c3e81d9: B -> in_B ; v_5c5313465744: C -> in_C ; f_b08deefb996d: at_most_one -> out_at_most_one (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 (declare-const in_B Bool) ; B (declare-const in_C Bool) ; C ; ---- Outputs (dependencies first) ; output at_most_one (stable ID f_b08deefb996d): NOT ((A AND B) OR (A AND C) OR (B AND C)) (define-fun out_at_most_one () Bool (not (or (and in_A in_B) (and in_A in_C) (and in_B in_C)))) ; ---- 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
at_most_one = \overline{\left(A \cdot B\right) + \left(A \cdot C\right) + \left(B \cdot C\right)}The common slip
Confusing “at most one” with “exactly one”. They agree on seven of the eight rows and differ on the all-false row, A = FALSE, B = FALSE, C = FALSE, where at-most-one is true and exactly-one is false. The row 0 column in the table is the quickest way to tell them apart.
Questions
Why do the NAND and NOR constructions need the same number of gates here?
Both converters report 19 gates for this expression, a negation wrapped around a majority. The counts describe each family's mechanical construction, replayed against the table, not the function itself; here the two constructions happen to come out the same size, which is not true of most pages.
Related
Operators and gates
- NOT gate (inverter): truth table, symbol and the NOT operator
- AND gate: truth table for 2, 3 and 4 inputs, symbol and expression
- OR gate: truth table for 2, 3 and 4 inputs, inclusive or explained
Tools
- Truth table generator: any Boolean expression, up to 12 variables, with intermediate columns
- Boolean algebra calculator and simplifier with steps (every law named and verified)
- Karnaugh map solver: 2 to 4 variables, groups, SOP and POS, don’t-cares from assumptions
See also
- Exactly one of three: why XOR is the wrong tool
- Majority of three (2-out-of-3 voting): truth table, K-map and NAND form
- De Morgan's laws: NOT (A AND B) = NOT A OR NOT B, with proof and examples
- CNF and DNF converter: conjunctive and disjunctive normal forms, minterms and maxterms
- Exactly one payment option: cash, card or voucher
- Conjunctive normal form (CNF): meaning and examples
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.