A combustion system having improved mercury collection. The histogram construction counts string-attribute pairs in the documents, and outputs string-attribute-count triples sorted by count. Various embodiments provide an apparatus with a multi-notch filter. The preamble can be an Ethernet preamble. Leading faces of the soil moving elements are oriented at opposed angles to the travel direction such that forward portions thereof are farther apart than rearward portions and such that soil contacted by each face is moved toward the shank rearward of the shank.