<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def for_account_hash(parent, account_hash):
 """ Returns a new AccountProxy that acquires the account with the given hash, if such an account is known to the account manager. It is an error if the account manager does not have such an account. """

        account = AccountProxy(parent)
        account.account_hash = account_hash
        if account.acquire():
            return account
        return None