Consider the sentences in stringr::sentences (loaded automatically with the tidyverse as sentences ). How many words appear once and only once in the entire set? For example, the word "The" definitely appears more than once, while the word "canoe" only appears one (in the first sentence). Hint: First extract all words and convert them to lowercase, so you get one long tibble like this (here I'm only showing the first 10 words): ## # A tibble: 10 x 1 word 1 the 2 birch 3 canoe 4 slid 5 on ## the 7 smooth 8 planks 9 glue ## 10 the