1. A securities processor comprising: a prematching process for validating an attributable security interest message placed by a market participant on a securities trading system, wherein the attributable security interest message relates to an individual security chosen from a plurality of securities traded on the securities trading system; a matching queue for receiving the validated attributable security interest message from the prematching process and for queuing the validated attributable security interest message for subsequent matching; and a matching process for obtaining the validated attributable security interest message from the matching queue and for comparing it with previously-validated attributable security interest messages to determine if a match occurs between the validated attributable security interest message and one of the previously-validated attributable security interest messages.