无法理解R代码段中发生的事情

Question

在以下代码中：

library(Lahman)
library(broom)
library(dplyr)

# load the baseball Teams data frame
data(Teams)

# make a copy of the Teams data frame
teams_tab <- Teams

# add stolen bases per game and runs per game columns
teams_tab <- teams_tab %>%  mutate(sb_per_game = SB / G, runs_per_game = R / G)

summary <- teams_tab %>% 
  filter(yearID >= 1961) %>%
  group_by(yearID) %>%
  do(tidy(lm(runs_per_game ~ sb_per_game, data = .)))

我不明白data = .函数中lm正在做什么。我也注意到do(tidy(lm(.$runs_per_game ~ .$sb_per_game)))也有效，但我不明白为什么。有人可以解释点运营商在做什么吗？