`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> = .