human
I wrote the below code

    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(expr)):
        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
        elif (not satisfiable(result)):
            return False
    return None

What's a good function header?
 Here's a good function header?
assistant
#Here's a good function header.

def pl_true(expr, model={}, deep=False):