overunder

Commands:

• load - loads the input text
• show - shows which line you are currently working on
• over + int(x) - keeps the corresponding x characters intact
• under + int(x) - replaces the corresponding x characters with a symbol
• pattern - generates the pattern created so far
• save - saves the pattern in a text file
• enter/return - moves to the next line of text
• quit - quits the program

See more here.