# ablm.py: parser based on
#     S --->  b M b
#     M --->  ( L  |  a
#     L --->  M a )

import sys
next = None
src = ""

def S ():
    if next != 'b':
        error(1)
    scan()
    M()
    if next != 'b':
        error(2)
    scan()

def M ():
    if next == '(':
        scan() 
        L()
    elif next == 'a':
        scan()
    else:
        error(3)

def L ():
    M()
    if next == 'a':
        scan()
        if next == ')':
            scan()
        else:
            error(4)
    else:
        error(5)
   
def error(n):
    sys.stdout.write("ERROR:" +
      str(n))
    sys.stdout.write(", SRC:" +
      src + "\n")
    sys.exit(1)
    
def getch():
    c = sys.stdin.read(1)
    if len(c) > 0:
        return c
    else:
        return None

def scan():
    global next
    global src
    next = getch()
    if next == None:
        sys.exit(1)
    while next.isspace(): # whitespace
        next = getch()
    src += next

scan()
S()
if next == '$':
    sys.stdout.write("Success! SRC=" +
      src + "\n")
else:
    error(0)

% python ablm.py
b(aa)b$
Success! SRC=b(aa)b$
a(a)b$
ERROR:1, SRC:a
b(((aa)a)a)b$
Success! SRC=b(((aa)a)a)b$
b((aa)a)a)b$
ERROR:2, SRC:b((aa)a)a
bab$
Success! SRC=bab$
baab$
ERROR:2, SRC:baa
b(ba)b$
ERROR:3, SRC:b(b
b((aaa)a)$
ERROR:4, SRC:b((aaa
b((aa)a)b$
Success! SRC=b((aa)a)b$
baba$
ERROR:0, SRC:baba
b(a)b$
ERROR:5, SRC:b(a)
b((((aa)a)a)a)b$
Success! SRC=b((((aa)a)a)a)b$
aba$
ERROR:1, SRC:a
b((ab)a)$
ERROR:5, SRC:b((ab