Frankly, I didn't find the explanation allthat enlightening.
    Is "a final arrow" more emphatic than "the final arrow"?  Am
    I right in recalling that we used to call "a" an "indefinite
    article" and "the" a "definite article"?  Does it make sense
    for a phrase beginning with an indefinite article to be more
    emphatic than one that begins with a definite article?
 
    Or, considering it another way.  The question was posed, I
    believe, by a "non-native" speaker of English--that is,
    someone who might have trouble in some or many cases choosing
    correctly between "a" and "the."  But here BOTH are correct;
    so, isn't the question, why can either one be used here, and
    what exactly is the difference created by the choice of one
    over the other?  And what in fact causes that difference?
    And what other instances of the same situation might be
    cited?
 
    Norm Carlson