'Returns a list of usernames of approvers who approved this approval.'

def GetNonExpiredApprovers(self):

    lifetime = rdfvalue.Duration((self.Get(self.Schema.LIFETIME) or config.CONFIG['ACL.token_expiry']))
    approvers = set()
    now = rdfvalue.RDFDatetime.Now()
    for approver in self.GetValuesForAttribute(self.Schema.APPROVER):
        if ((approver.age + lifetime) > now):
            approvers.add(utils.SmartStr(approver))
    return list(approvers)
