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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:

def letter():
    '''Parse a letter in alphabet.'''
    @Parser
    def letter_parser(text, index=0):
        if index < len(text) and text[index].isalpha():
            return Value.success(index + 1, text[index])
        else:
            return Value.failure(index, 'a letter')
    return letter_parser