lex
	literal 'var', 'if', 'then', 'else', 'while', 'do', 'for', 'read', 'write',
			'end', 'to', 'goto'
	literal ':=', '!=', ';', '+', '-', '*', '/', '=', '(', ')', ':'

	ignore /'//' [^\n]* '\n'/
	ignore /[\n\t ]+/
	token id /[a-zA-Z_]+/
	token integernumber /[0-9]+/
	token stringlit /'"' [^"]* '"'/
end

def program
	[statement*]

def statement
	[declaration]
|	[assignment_statement]
|	[if_statement]
|	[while_statement]
|	[do_statement]
|	[for_statement]
|	[read_statement]
|	[write_statement]
|	[labelled_statement]
|	[goto_statement]

def declaration
	['var' id ';']

def assignment_statement
	[id ':=' expression ';']

def if_statement
	['if' expression 'then' statement* opt_else_statement 'end']

def opt_else_statement
	['else' statement*]
|	[]

def while_statement
	['while' expression 'do' statement* 'end']

def do_statement
	['do' statement* 'while' expression ';']

def for_statement
	['for' id ':=' expression 'to' expression 'do' statement* 'end']

def read_statement
	['read' id ';']

def write_statement
	['write' expression ';']

def expression
	[term]
|	[expression eqop term]

def eqop ['='] | ['!=']

def term
	[factor]
|	[term addop factor]

def addop ['+'] | ['-']

def factor
	[primary]
|	[factor mulop primary]

def mulop ['*'] | ['/']

def primary
	[id]
|	[lit]
|	['(' expression ')']

def lit
	[integernumber]
|	[stringlit]

def labelled_statement
	[id ':' statement] 

def goto_statement
	['goto' id ';']

parse PP: program[stdin]
P: program = PP.tree

#for S:statement* in P
#{
#	if match S [L0: id ':' 
#			First: statement 
#			Rest: statement*]
#	{
#		for Check: statement* in Rest
#		{
#			if match Check 
#				['if' E: expression 'then' 
#					'goto' Targ: id ';'
#				'end'
#				T: statement*]
#			{
#				# This truncates Rest
#				Check = construct statement* []
#
#				# Replace the labeled statement through to the goto with a
#				# do ... while.
#				S = construct statement*
#					['do'
#						First
#						Rest
#					'while' E ';'
#					T]
#				break
#			}
#		}
#	}
#}

for S: statement* in P
{
    if match S [Label: id ':' 
            First: statement 
            Rest: statement*]
    {
		Expr: expression
		Following: statement*

		# Look though the remaining statements for a goto back to the label.
		# The repeat iterator yields only top-level statement lists. It
		# restricts our search to the same nesting depth as the label.
        for Check: statement* in Rest
        {
            if match Check 
                ['if' E: expression 'then' 
                    'goto' L:id ';'
                'end'
                SL: statement*]
            {
				Expr = E
				Following = SL

				# Check iterates over tails of Rest. Assigning an empty list
				# to check truncates the Rest list. What we cut off is saved in
				# Following (excluding the if statement).
                Check = construct statement* []
            }
        }

		# If a goto was found, then perform the rewrite.
		if ( Expr )
		{
            # Replace the labelled statement through to the goto 
			# with a do ... while.
            S = construct statement* [
				"do
                "	[^First]
                "	[^Rest]
                "while [^Expr];
                Following]
		}
    }
}

print( %P )