Add answers to Blankfill, Reading, TextComp, Matching and Writing.
Remove prompt from selection definition.
Add visitor interface and pretty printer
Remove comments.
Add code for counting blanks.
Add comments in Main.java
Reduce 14 shift-reduce conflict by removing answer_list_opt.
Improve pretty printer.
Simplify production comments.
Replace StringBuilder with PrintStream in PrettyPrinter.
Added validator for performing basic validation.
Added skeleton file for db writing.
Fixed blank counting bug.
Add number of blanks in pretty printing.
Show warning message when number of selection items is not 4.
Show result message after validation.
Tagging standalone A, B, C, D as Chinese.
Item may have a space between the captical letter and dot.
Remove - ahead of english sentence.
Finished filling answer of each node by rewriting the AST.