For this assignment, you are to extend your SILLY interpreter to handle counter-driven loops, input statements, and simple subroutines. The expanded grammar rules for the SILLY language are as follows:

As stated in HW3, variables are case sensitive, need not be declared, and are assumed to have value 0 if not otherwise assigned. You are to make the following additions:

• Add a new repeat statement that performs counter-driven repetition. The keyword "repeat" is followed by a number or expression that specifies the number of repetitions. The sequence of statements that precede "end" are to be executed that number of times. If the number specified is zero or negative, the statements will not be executed at all.
• Add a new input statement that reads a number from standard input. The keyword "input" is followed by a variable name, and the number entered by the user (from standard input) is to be stored in that variable.
• Add simple subroutine declarations that can appear at the beginning of the program. A subroutine begins with the keyword "subroutine" followed by its name and zero or more parameters enclosed in parentheses. Note: you can assume that spaces will surround the parentheses to make parsing easier. Next, there can be a sequence of local variable declarations, each of which consists of the keyword "local" followed by a variable name. Local variables exist only inside the subroutine, and can override global variables with the same names. Finally, the sequence of statements that are to be executed as a result of calling the subroutine follow, terminated by the keyword "end". Static scoping is assumed, so any variables referenced within these statements that are not parameters or declared local are assumed to be global variables.
• Finally, add a new subroutine call statement. A call begins with the keyword "call" followed by the name of the subroutine and a series of expressions contained in parentheses. Again, you may assume that spaces will surround the parentheses to make parsing easier. By-value parameter passing is assumed, so the values of the expressions are passed to the subroutine and copied into the parameters (which behave the same as local variables).

As before, your SILLY interpreter should read the program from a file (as specified by the user) and display the output that would be produced by the SILLY program. If there are any errors in the program, e.g., calling an undefined subroutine or specifiying the wrong number of parameters in the call, the interpreter should display "SYNTAX ERROR" and halt. For example:

SAMPLE PROGRAM OUTPUT x = 3 repeat x + 1 output "foo" end input y while y output y y = y - 1 end foo foo foo foo 3 2 1

SAMPLE PROGRAM OUTPUT subroutine s1 ( ) output "inside s1" output a end subroutine s2 ( x ) local a local b a = x + 1 b = a + 1 output "inside s2" output a output b call s1 ( ) end a = 5 call s2 ( a ) output "done" inside s2 6 7 inside s1 5 done

You are strongly encouraged to add language features incrementally. Credit will be awarded based on the following guidelines:

25%	repeat statement
25%	input statement
20%	subroutines & calls with no local variables or parameters
15%	subroutines & calls with local variables
15%	subroutines & calls with parameters