编写要求：
MACD的DIF向上和向下拐头的预警选股公式
公式在线网解答：
DIF:=EMA(CLOSE,12)-EMA(CLOSE,26); XG:(DIF>REF(DIF,1) AND REF(DIF,1)<REF(DIF,2)) OR (DIF<REF(DIF,1) AND REF(DIF,1)>REF(DIF,2));
