This is still unclear. To which part(s) of the sentence does "but only deleting items before the first element of S or after the last element of S" refer? To the entire preceding part of the sentence, or only to the part after "deleting some items"? More to the point, given the following two sequences:

@seq1 = qw(a b c d e f g h);
@seq2 = qw(a b c e f x g h);
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is the answer  a b c  or  a b c e f ?