setClass("oblig", slots = c(begin = "Date"), contains = "Date")
new("oblig", as.Date("2014-01-01"))
程序运行报错，请问这种情况应该如何处理
Error in validObject(.Object) :
invalid class “oblig” object: invalid object for slot "begin" in class "oblig": got class "S4", should be or extend class "Date"
