If the operation requires testing sequence elements according to some criterion, then the criterion may be specified in one of two ways. The basic operation accepts an item, and elements are tested for being eql to that item. This searches for the first element of sequence whose car is eq to item . For some operations it can be useful to specify the direction in which the sequence is conceptually processed. Depending on the object to be processed and on the implementation, the actual processing order may be different.
Many operations allow the specification of a subsequence to be operated upon.
They indicate the subsequence starting with and including element start and up to but excluding element end . The length of the subsequence is therefore end - start . Therefore if both start and end are omitted, the entire sequence is processed by default. This may be summarized as follows. Let x be the sequence within which indices are to be considered.