7. A computer comprising: an editing module programmed to execute a method of editing a set of rules contained in a first document, said rules defining a transformation of data on a second document, said first document and said second document having elements constructed in a tree structure having nodes, each said rule on said first document having a template structure including at least one match condition, said match condition comprising an expression in said template that describes a subset of tree nodes, said editing module comprising: an input module for receiving a selection of at least one rule in said first document; a comparator for checking a match condition of said at least one selected rule with said tree nodes from said second document to determine which structures in said second document match said selected rule; and a display for presenting a listing of proposed rule modifications based on said determination of matched structures, said rule modifications used to provide said edited set of rules, wherein said edited set of rules defines a transformation of said data on said second document to produce a resulting output document having a structure determined exclusively by said edited set of rules, and by the structure of said second document, and wherein any rules having an identical effect are merged into a single rule.
7. A computer comprising: an editing module programmed to execute a method of editing a set of rules contained in a first document, said rules defining a transformation of data on a second document, said first document and said second document having elements constructed in a tree structure having nodes, each said rule on said first document having a template structure including at least one match condition, said match condition comprising an expression in said template that describes a subset of tree nodes, said editing module comprising: an input module for receiving a selection of at least one rule in said first document; a comparator for checking a match condition of said at least one selected rule with said tree nodes from said second document to determine which structures in said second document match said selected rule; and a display for presenting a listing of proposed rule modifications based on said determination of matched structures, said rule modifications used to provide said edited set of rules, wherein said edited set of rules defines a transformation of said data on said second document to produce a resulting output document having a structure determined exclusively by said edited set of rules, and by the structure of said second document, and wherein any rules having an identical effect are merged into a single rule. 9. The computer of claim 7 , further comprising: an input module to receive a selection of a preferred one of said choice of proposed rule modifications; and a rule modification module to modify said listing of proposed rule modifications, based on said selection.