Implement the un-implemented functions to make a working parser.
An example of a parse from a properly working program would be:
 Enter expression: bcac S 2 AB 3 bAB 4 bcB 5 bcaA 4 bcac bcac | $

''' This program implements a recursive descent parser for the CFG below: 1. S  aA 2. S  AB 3. A  bA 4. A  c 5. B  aA 6. B  b ''' import math class ParseError(Exception): pass w = '' # the input string i = 0 # index of the next (lookahead) token p = 'S' # the current sentential form in the derivation def read_token(): global i i += 1 def look_ahead(): return w[i] #============================================================== # FRONT END PARSER #============================================================== def S(): if look_ahead() == 'a': rule_1() read_token() A() elif look_ahead() in ('b', 'c'): rule_2() A() B() else: raise ParseError def A(): # implement def B(): # implement #============================================================== # BACK END PARSER (ACTION RULES) #============================================================== def rule_1(): global p print('1') p = p.replace('S', 'aA', 1) print(p) def rule_2(): # implement def rule_3(): # implement def rule_4(): # implement def rule_5(): # implement def rule_6(): # implement #============================================================== # User Interface Loop #============================================================== w = input(' Enter expression: ') while w != '': i = 0 w = w + '$' # EOF marker print('S') try: S() # call the parser except: if look_ahead() == '$': print('parse error: unexpected EOF') else: print('parse error: unexpected symbol') print(w[:i], '|', w[i:]) w = input(' Enter expression: ')