I would like to filter to remove all rows before the first time a particular value in a specific column appears. For example, in the data frame below, I would like to remove all rows before `bob` appears in column `a` for the first time. Please note that the value of `bob` repeats a second time -I only want to remove the rows before the first time `bob` appears.

```
(dat<-data.frame(a= c("pete", "mike", "bob", "bart", "bob"), b=c(1,2,3,4,5), c=c("home", "away", "home", "away", "gone")))
     a b    c
1 pete 1 home
2 mike 2 away
3  bob 3 home
4 bart 4 away
5  bob 5 gone

```

I want the resulting data frame to look like the following:

```
   a   b  c
1 bob  3 home
2 bart 4 away
3 bob  5 gone

```