Xerox Corp has been threatening to eat its own children by obsoleting the copier with multifunction laser printers that combine facsimile and scanning with copying for five years, and yesterday it finally brought the mass market products down to the wire. The initial offerings in the new line, believed to be Sparc-driven, are the Xerox Document Centre System 20 and the Document Centre System 35. Both have scanners for copying and printing and can be driven from a personal computer or from the printer console. The System 20 is out now at $19,500 and the System 35 ships later this quarter at $38,610. There is also a Document WorkCentre 250 targeted at the home and small office market, which also combines printing, scanning, copying and facsimile in one device, for $700.