mlogit(formula = vcho ~ ideodiff + welfspddiff + defspddiff | nat.econ.futr + age + income + educ + gender + hometown, data = data.mlogit.cand, alt.subset = c("ahn", "moon", "hong"), reflevel = "ahn", method = "nr", print.level = 0)，请问大家这个模型里的"|"是什么意思啊？