Parallel Letter Frequency

Files: occurrences.go, occurrences_test.go

Count the frequency of letters in texts using parallel computation.

Parallelism is about doing things in parallel that can also be done sequentially.
A common example is counting the frequency of letters.
Create a function that returns the total frequency of each letter in a text and that employs parallelism.

The implementation depends on the following packages:

• btree: from github.com/emirpasic/gods/trees/btree (https://github.com/emirpasic/gods ); you have to go get github.com/emirpasic/gods

• alphabet: see https://github.com/massimo-marino/alphabet.git