1. A tool in a computer system for graphically defining an expression, the tool comprising: a graphical user interface component operable to create a graphical definition of the expression based on one or more tree structures, each of the one or more tree structures comprising a plurality of nodes and being associated with one or more lists, wherein each of the one or more lists comprises a plurality of items, each of the plurality of items being associated with a node of the tree structure associated with the respective list; and an expression generator component coupled in communication to the graphical user interface component, the expression generator component being operable to generate the expression based on the graphical definition created by the graphical user interface component, wherein the expression is adapted to filter or modify one or more messages published by a publisher application prior to delivery of the one or more messages to one or more subscriber applications, the expression being adapted to modify at least one of the one or more messages by merging data from one or more databases into the at least one message.