aima/logic.py
def unit_clause_assign(clause, model):
    """Return a single variable/value pair that makes clause true in
    the model, if possible.
    >>> unit_clause_assign(A|B|C, {A:True})
    (None, None)
    >>> unit_clause_assign(B|~C, {A:True})
    (None, None)
    >>> unit_clause_assign(~A|~B, {A:True})
    (B, False)
    """
    P, value = None, None
    for literal in disjuncts(clause):
        sym, positive = inspect_literal(literal)
        if sym in model:
            if model[sym] == positive:
                return None, None  # clause already True
        elif P:
            return None, None      # more than 1 unbound variable
        else:
            P, value = sym, positive
    return P, value
def unit_clause_assign(clause, model):
    """Return a single variable/value pair that makes clause true in
    the model, if possible.
    >>> unit_clause_assign(A|B|C, {A:True})
    (None, None)
    >>> unit_clause_assign(B|~C, {A:True})
    (None, None)
    >>> unit_clause_assign(~A|~B, {A:True})
    (B, False)
    """
    P, value = None, None
    for literal in disjuncts(clause):
        sym, positive = inspect_literal(literal)
        if sym in model:
            if model[sym] == positive:
                return None, None  # clause already True
        elif P:
            return None, None      # more than 1 unbound variable
        else:
            P, value = sym, positive
    return P, value
Return a single variable/value pair that makes clause true in
    the model, if possible.
    >>> unit_clause_assign(A|B|C, {A:True})
    (None, None)
    >>> unit_clause_assign(B|~C, {A:True})
    (None, None)
    >>> unit_clause_assign(~A|~B, {A:True})
    (B, False)