If you've ever written up some BNF and wanted a parser for it, you've encountered the reality: No standard tool generated a parser from arbitrary BNF. BNF had to be put into special and highly restrictive forms like LALR or LL, and this usually required a lot of tweaking of the grammar. Marpa eliminates these restrictions. It will generate a parser for any BNF. That includes even BNF for grammars which are infinitely ambiguous.
Marpa was the greatest of the Tibetan "translators". In Marpa's time (the 11th century AD) Indian Buddhism was at its height. A generation of scholars was devoting itself to producing Tibetan versions of Buddhism's Sanskrit scriptures. Marpa became the greatest of them, and today is known as Marpa Lotsawa: "Marpa the Translator".
Translation in the 11th century was not a job for the indoors type. A translator needed to study in India, with the teachers who had the texts and could explain them. From Marpa's home in Tibet's Lhotrak Valley, the best way across the Himalayas to India was over the Khala Chela Pass. To reach the Khala Chela's three-mile high summit, Marpa had to cross two hundred lawless miles of Tibet. Once a pilgrim crested the Himalayas, the road to Nalanda University was all downhill. Eager to reach their destination, the first travelers from Tibet had descended the four hundred miles straight to the hot plains.
The last part of the journey had turned out to be by far the most deadly. Almost no germs live in the cold, thin air of Tibet. Pilgrims who didn't stop to acclimatize themselves reached the great Buddhist center with no immunity to India's diseases. Several large expeditions reached Nalanda only to have every single member die within weeks.
There's more about Marpa in my novel, The God Proof, in which his studies, travels and adventures are a subplot. The God Proof centers around Kurt Gödel's proof of God's existence. Yes, that Kurt Gödel, and yes, he really did work out a God Proof (it's in his Collected Works, Vol. 3, pp. 403-404). The God Proof is available as a free download (http://www.lulu.com/content/933192) and in print form at Amazon.com: http://www.amazon.com/God-Proof-Jeffrey-Kegler/dp/1434807355.