In the example dataset below I need to find the number of unique customers per product summarised per year. The output has to be a data.frame with the headers: **year - product - number of customers**

Thanks for your help.

```
year <- c("2009", "2010")
product <- c("a", "b", "c")
df <- data.frame(customer = sample(letters, 50, replace = T),
                 product = sample(product, 50, replace = T),
                 year = sample(year, 50, replace = T))

```