Subseq returns a subsequence of attributes from the Rest of the Rest of the path. The first element of the path is a positive integer that specifies the start point of the subsequence (counting from 0) and the second element of the path is a (possibly negative) integer that specifies the length of the subsequence. If the either first path element or the sum of the first and second path elements is not a positive integer or if the path is too short to include the specified subsequence then Subseq fails.
Subseq:<0 1 a b c d e f> = a.
Subseq:<1 2 a b c d e f> = b c.
Subseq:<2 3 a b c d e f> = c d e.
Subseq:<5 1 a b c d e f> = f.
Subseq:<5 -1 a b c d e f> = e.
Subseq:<5 0 a b c d e f> = .
