> s[1][1]
$call
lm(formula = TAX ~ GDP, data = data)

> s[1][1][1]
$call
lm(formula = TAX ~ GDP, data = data)

> s[1][1][1][1][1]
$call
lm(formula = TAX ~ GDP, data = data)



为什么是s[1][1][1].....会不断地延续下去？