Abdessamad Echihabi Ulf Hermjakob Eduard H. Hovy Daniel Marcu Eric Melz Deepak Ravichandran Multiple-Engine Question Answering in TextMap. 772-781 2003 TREC http://trec.nist.gov/pubs/trec12/papers/usc-isi.qa.pdf db/conf/trec/trec2003.html#EchihabiHHMMR03