human
Solve the following python programming problem given the following code:
def pl_true(expr, model = { 
    
}, deep = False) : 
    "
    Returns whether the given assignment is a model or not.

    If the assignment does not specify the value for every proposition,
    this may return None to indicate 'not obvious'.

    Parameters
    ==========

    model : dict, optional, default: {}
        Mapping of symbols to boolean values to indicate assignment.
    deep: boolean, optional, default: False
        Gives the value of the expression under partial assignments
        correctly. May still return None to indicate 'not obvious'.

    Examples
    ========

    >>> from sympy.abc import A, B, C
    >>> from sympy.logic.inference import pl_true
    >>> pl_true(A & B, {A: True, B: True})
    True
    >>> pl_true(A & B, {A: False})
    False
    >>> pl_true(A & B, {A: True})
    >>> pl_true(A & B, {A: True}, deep=True)
    >>> pl_true(A >> (B >> A))
    >>> pl_true(A >> (B >> A), deep=True)
    True
    >>> pl_true(A & ~A)
    >>> pl_true(A & ~A, deep=True)
    False
    >>> pl_true(A & B & (~A | ~B), {A: True})
    >>> pl_true(A & B & (~A | ~B), {A: True}, deep=True)
    False
    " 
    from sympy.core.symbol import Symbol 
    from sympy.logic.boolalg import BooleanFunction 
    boolean = (True, False) 
    def _validate(expr) : 
        if (isinstance (expr, Symbol) or (expr in boolean)) : 
            return True 
if (not isinstance (expr, BooleanFunction)) : 
            return False 
return all ((_validate (arg) for arg in expr.args)) 
if (expr in boolean) : 
        return expr 
expr = sympify (expr) 
    if (not _validate (k)) : 
        raise ValueError (('%s is not a valid boolean expression' % expr)) 
model = dict (((k, v) for (k, v) in model.items () if (v in boolean))) 
    result = expr.subs (model) 
    if (result in boolean) : 
        return bool (result) 
if deep : 
        model = dict (((k, True) for k in result.atoms ())) 
        if pl_true (result, model) : 
            if valid (result) : 
                return True 
else : 
            if (not...

    
    

    Fix the buggy line: if (not _validate (k)) :
assistant
#Here is Python program to fix the buggy line: if (not _validate (k)) :
.

if (not _validate (expr)) :