Dear R community;

I'm kind of stuck with the following situation and would appreciate any
hint. Let's assume I have the following data frame:

dat <- data.frame(ID = c(rep("01",18), rep("02",16)), USE = c(c("001","004",
"005","007","001","004","005","007","012","001","004","005","007","001","004",
"005","007","012"),c("004","005","007","013","001","004","005","007","001","004",
"005","007","001","004","005","007")),ID2 = "")

What I would like to achieve is to number all the "001" occurrences in USE
for each ID individually and store them in ID2. In other words, something
like this:

   ID USE ID2
1  01 001  1
2  01 004
3  01 005
4  01 007
5  01 001   2
6  01 004
7  01 005
8  01 007
9  01 012
10 01 001   3
11 01 004
12 01 005
13 01 007
14 01 001   4
15 01 004
16 01 005
17 01 007
18 01 012
19 02 004
20 02 005
21 02 007
22 02 013
23 02 001    1
24 02 004
25 02 005
26 02 007
27 02 001    2
28 02 004
29 02 005
30 02 007
31 02 001    3
32 02 004
33 02 005
34 02 007

Thanks again,
Pedro

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