看了Formula的文档，基本没看懂
Such a model consists of a series of terms separated by + operators. The terms themselves consist of variable and factor names separated by : operators. Such a term is interpreted as the interaction of all the variables and factors appearing in the term.
y~x1+x2
代表了y由x1和x2两个解释变量决定
但是
y~x1:x2是啥意思呢，请问，非常感谢
