typo corrected, "A Backward Glance O'er Travel'd Roads"

Mario Corona identified a transcription typo (http://whitmanarchive.org/published/LG/1891/poems/399) on page 429 of "A Backward Glance O'er Travel'd Roads" from Whitman's 1891-92 Leaves of Grass. "Just as all the old imaginative works rest, after their kind, on long trains of presuppositions, often entirely unmention'd by themselves, yet [TYPO: vet] supplying the most important bases of them..." ~Katie Kruger