A scan through nine documents, searching for English letters, produced the table below:
![](https://book.transtutors.com/qimg/931ad6a6-fcf4-470f-86ae-987fffc9ab08.png)
Make an alphabetical index for the terms by the following methods:
(a) Construction of an incidence matrix followed by construction of the inverted file.
(b) A sort and merge procedure.