<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def find_killers(self, var_def, simplified_graph=True):
 """ Find all killers to the specified variable definition. :param ProgramVariable var_def: The variable definition. :param bool simplified_graph: True if we want to search in the simplified graph, False otherwise. :return: A collection of all killers to the specified variable definition. :rtype: list """


        if simplified_graph:
            graph = self.simplified_data_graph
        else:
            graph = self.data_graph

        if var_def not in graph:
            return []

        killers = []
        out_edges = graph.out_edges(var_def, data=True)
        for _, dst, data in out_edges:
            if 'type' in data and data['type'] == 'kill':
                killers.append(dst)

        return killers