This program takes input.txt
It has unwanted (1234567890) and so on.
It only saves these letters:
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z a b c d e f g h i j k l m n o p q r s t u v w x y z
(space button)
(enter button) and (tab button) is converted to (space button)
Now collapse consecutive whitespaces into one space.
The others are removed.
Then the output.txt is created.