def __replace_an(sentence):
if sentence is not None:<EOL><INDENT>while sentence.find('<STR_LIT>') != -<NUM_LIT:1>:<EOL><INDENT>an_index = sentence.find('<STR_LIT>')<EOL>if an_index > -<NUM_LIT:1>:<EOL><INDENT>an_index += <NUM_LIT:4><EOL>if sentence[an_index] in '<STR_LIT>':<EOL><INDENT>sentence = sentence.replace('<STR_LIT>', str('<STR_LIT>'), <NUM_LIT:1>)<EOL><DEDENT>else:<EOL><INDENT>sentence = sentence.replace('<STR_LIT>', str('<STR_LIT:a>'), <NUM_LIT:1>)<EOL><DEDENT><DEDENT>if sentence.find('<STR_LIT>') == -<NUM_LIT:1>:<EOL><INDENT>return sentence<EOL><DEDENT><DEDENT>return sentence<EOL><DEDENT>else:<EOL><INDENT>return sentence<EOL><DEDENT>
Lets find and replace all instances of #AN
    This is a little different, as this depends on whether the next
    word starts with a vowel or a consonant.

    :param _sentence: