Consensus in practice: AB + A'C + BC simplified
(A AND B) OR (NOT A AND C) OR (B AND C) compared with the same expression minus its third term. The equivalence check proves them equal on eight rows, the simplifier leaves the long form alone, and the Karnaugh map drops the consensus term.
The third term, B AND C, is the consensus of the first two: take the terms that contain A and NOT A, delete that variable, and AND what is left. Whenever B AND C is true, either A is true and A AND B already holds, or A is false and NOT A AND C already holds. The engine's reading lists the three conditions as an inclusive OR. On the eight-row table both formulas are true on the same four rows, Σm(1, 3, 6, 7).
The equivalence check holds: with_consensus and without agree on every assignment. Not a tautology, counterexample A = FALSE, B = FALSE, C = FALSE; satisfiable, witness A = FALSE, B = FALSE, C = TRUE. The simplifier found no law that shortens the six operators, since no single law removes a consensus term; the DNF is unchanged. The CNF, (A OR C) AND (NOT A OR B) AND (B OR C), keeps a consensus clause too. The Karnaugh map prunes: three prime implicants, but two groups cover every 1, deriving (A AND B) OR (NOT A AND C). Gates: 11 NAND, 35 NOR.
Consensus terms creep into rules when two people write the same policy from different angles: “admins who are active” plus “non-admins with a pass” plus, later, “anyone active with a pass”. The third clause is harmless but misleading, because readers assume every clause matters. In hardware the opposite care applies: the redundant term is sometimes kept on purpose to cover a timing hazard when A switches. The thing to watch is which tool you trust for pruning: the map found the shorter cover, the simplifier did not, and the equivalence check is what makes the shorter form safe to adopt.
with_consensus := (A AND B) OR (NOT A AND C) OR (B AND C) · outputwithout := (A AND B) OR (NOT A AND C) · output
Exact checks
Truth table (8 rows)
| # | A | B | C | with_consensus | without |
|---|---|---|---|---|---|
| 0 | FALSE | FALSE | FALSE | FALSE | FALSE |
| 1 | FALSE | FALSE | TRUE | TRUE | TRUE |
| 2 | FALSE | TRUE | FALSE | FALSE | FALSE |
| 3 | FALSE | TRUE | TRUE | TRUE | TRUE |
| 4 | TRUE | FALSE | FALSE | FALSE | FALSE |
| 5 | TRUE | FALSE | TRUE | FALSE | FALSE |
| 6 | TRUE | TRUE | FALSE | TRUE | TRUE |
| 7 | TRUE | TRUE | TRUE | TRUE | TRUE |
Plain-language reading
Beginnerwith_consensus is true exactly when at least one of the following holds (several may hold):
1. both A and B are true
2. A is false and C is true
3. both B and C are true
In every other case, with_consensus is false.
About the operators:
- 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.
- NOT reverses a value: true becomes false and false becomes true.
Intermediatewith_consensus is true exactly when at least one of the following holds (several may hold):
1. both A and B are true
2. A is false and C is true
3. both B and C are true
In every other case, with_consensus is false.
Advancedwith_consensus is true exactly when at least one of the following holds (several may hold) [inclusive disjunction ∨]:
1. both A and B are true [conjunction ∧]
2. A is false and C is true [conjunction ∧]
3. both B and C are true [conjunction ∧]
In every other case, with_consensus 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.
(A AND B) OR (NOT A AND C) OR (B AND C)
Karnaugh map
- cells m6, m7
A AND B - cells m1, m3
NOT A AND C
Minterms and maxterms
with_consensus(A, B, C) = Σm(1, 3, 6, 7)with_consensus(A, B, C) = ΠM(0, 2, 4, 5)Variable order: A, B, C · 8 rows
Normal forms
Conjunctive normal form (CNF)
(A OR C) AND (NOT A OR B) AND (B OR C)3 clauses, 6 literals · complete · Distribution ×2, Complement ×1, Idempotence ×1, Absorption ×2
Disjunctive normal form (DNF)
(A AND B) OR (NOT A AND C) OR (B AND C)3 terms, 6 literals · complete
NAND-only and NOR-only
NAND-only form
NAND(NAND(NAND(NAND(A, B), NAND(NAND(A, A), C)), NAND(NAND(A, B), NAND(NAND(A, A), C))), NAND(B, C))11 gates · verified equivalent on every row
NOR-only form
NOR(NOR(NOR(NOR(NOR(NOR(A, A), NOR(B, B)), NOR(A, NOR(C, C))), NOR(NOR(NOR(A, A), NOR(B, B)), NOR(A, NOR(C, C)))), NOR(NOR(B, B), NOR(C, C))), NOR(NOR(NOR(NOR(NOR(A, A), NOR(B, B)), NOR(A, NOR(C, C))), NOR(NOR(NOR(A, A), NOR(B, B)), NOR(A, NOR(C, C)))), NOR(NOR(B, B), NOR(C, C))))35 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: 8be5b9c2d9a2e4d9023409a6a462a5601cb2eaaec1809bee846aee48212e1504
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:
'with_consensus' -> output_with_consensus(inputs)
'without' -> output_without(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})
# -> {'with_consensus': False, 'without': False}
model.output_with_consensus({'A': False, 'B': False, 'C': False})
# -> False
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_with_consensus",
"output_without",
]
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": "8be5b9c2d9a2e4d9023409a6a462a5601cb2eaaec1809bee846aee48212e1504",
"assumptions_digest": "d58ee1ad38e23e5093e4baa1047b5ae70d6972e08f555e34ac3fa16df63c4a78",
"primary_output": "with_consensus",
"unused_inputs": (),
"subcircuits": (),
}
# Stable ID -> generated identifier (reversible mapping; names are exact, escaped strings).
IDENTIFIER_MAP = {
"v_3a51a5515ad0": {"kind": "input", "name": "A", "identifier": "input_A"},
"v_f16c47ed4d2f": {"kind": "input", "name": "B", "identifier": "input_B"},
"v_94adb63762fe": {"kind": "input", "name": "C", "identifier": "input_C"},
"f_008d5f612658": {"kind": "output", "name": "with_consensus", "identifier": "output_with_consensus"},
"f_84a7e6c2ece2": {"kind": "output", "name": "without", "identifier": "output_without"},
}
INPUT_NAMES = ("A", "B", "C")
OUTPUT_NAMES = ("with_consensus", "without")
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_with_consensus(input_A: bool, input_B: bool, input_C: bool) -> bool:
"""Decision output 'with_consensus': (A AND B) OR (NOT A AND C) OR (B AND C)"""
return (input_A and input_B) or ((not input_A) and input_C) or (input_B and input_C)
def _compute_output_without(input_A: bool, input_B: bool, input_C: bool) -> bool:
"""Decision output 'without': (A AND B) OR (NOT A AND C)"""
return (input_A and input_B) or ((not input_A) and input_C)
# ---- Public API ----
def output_with_consensus(inputs: Mapping[str, bool], *, check_scope: bool = True) -> bool:
"""Decision output 'with_consensus' (stable ID f_008d5f612658).
'with_consensus' := (A AND B) OR (NOT 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_with_consensus_value = _compute_output_with_consensus(values["A"], values["B"], values["C"])
return output_with_consensus_value
def output_without(inputs: Mapping[str, bool], *, check_scope: bool = True) -> bool:
"""Decision output 'without' (stable ID f_84a7e6c2ece2).
'without' := (A AND B) OR (NOT A 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_without_value = _compute_output_without(values["A"], values["B"], values["C"])
return output_without_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_with_consensus_value = _compute_output_with_consensus(values["A"], values["B"], values["C"])
output_without_value = _compute_output_without(values["A"], values["B"], values["C"])
return {
"with_consensus": output_with_consensus_value,
"without": output_without_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: 8be5b9c2d9a2e4d9023409a6a462a5601cb2eaaec1809bee846aee48212e1504
-- 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_3a51a5515ad0: A -> "in_A"
-- v_f16c47ed4d2f: B -> "in_B"
-- v_94adb63762fe: C -> "in_C"
-- Output columns:
-- f_008d5f612658: with_consensus -> "out_with_consensus"
-- f_84a7e6c2ece2: without -> "out_without"
-- 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_with_consensus] output with_consensus (stable ID f_008d5f612658)
-- with_consensus := (A AND B) OR (NOT A AND C) OR (B AND C)
-- PRECONDITION: "in_A", "in_B", "in_C" are non-NULL booleans.
-- (("in_A" AND "in_B") OR ((NOT "in_A") AND "in_C") OR ("in_B" AND "in_C"))
--
-- [out_without] output without (stable ID f_84a7e6c2ece2)
-- without := (A AND B) OR (NOT A AND C)
-- PRECONDITION: "in_A", "in_B", "in_C" are non-NULL booleans.
-- (("in_A" AND "in_B") OR ((NOT "in_A") 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 (("in_A" AND "in_B") OR ((NOT "in_A") AND "in_C") OR ("in_B" AND "in_C")) END AS "out_with_consensus",
CASE WHEN "status" = 'ok' THEN (("in_A" AND "in_B") OR ((NOT "in_A") AND "in_C")) END AS "out_without"
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: 8be5b9c2d9a2e4d9023409a6a462a5601cb2eaaec1809bee846aee48212e1504 ; 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_3a51a5515ad0: A -> in_A ; v_f16c47ed4d2f: B -> in_B ; v_94adb63762fe: C -> in_C ; f_008d5f612658: with_consensus -> out_with_consensus ; f_84a7e6c2ece2: without -> out_without (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 with_consensus (stable ID f_008d5f612658): (A AND B) OR (NOT A AND C) OR (B AND C) (define-fun out_with_consensus () Bool (or (and in_A in_B) (and (not in_A) in_C) (and in_B in_C))) ; output without (stable ID f_84a7e6c2ece2): (A AND B) OR (NOT A AND C) (define-fun out_without () Bool (or (and in_A in_B) (and (not in_A) 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
with_consensus = \left(A \cdot B\right) + \left(\overline{A} \cdot C\right) + \left(B \cdot C\right)
without = \left(A \cdot B\right) + \left(\overline{A} \cdot C\right)The common slip
Expecting the simplifier to remove B AND C. It applies named laws one term at a time, and no single law deletes a consensus term, so it reports the expression unchanged. The map and the equivalence check together show the term is redundant: the two-term form matches on all eight rows.
Questions
Why does the map say three prime implicants when it uses two?
B AND C is a prime implicant in its own right: it covers cells 3 and 7 and cannot be enlarged. But those cells are already covered by the other two groups, so the cover does not need it. Prime implicants are candidates; the derived expression uses the ones required to cover every 1.
Is the two-term form always preferable?
On the table, yes: it is the same function with fewer gates to build. In a physical circuit the consensus term can be deliberately kept to avoid a glitch when A changes while B and C are both true. That is an electrical reason, not a logical one, and the model cannot see it.
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
- Consensus theorem: AB + A'C + BC = AB + A'C, with proof
- Prime implicants and essential prime implicants on a K-map
- Logical equivalence checker: two expressions, exact verdict, counterexample
- “Simplified” is not “minimal”: what a simplifier can promise
- Absorption law: A OR (A AND B) = A, with proof
- 2-to-1 multiplexer: (S AND B) OR (NOT S AND A)
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.