1. A method for graphical specification of an XML to XML transformation rule, the method comprising the steps of: receiving a user's selection of a user-selected, optionally ambiguous, target leaf node, from a target tree of an initial target context, the target leaf node representing a target XML element to be mapped by the transformation rule; if the target leaf node is ambiguous, disambiguating the target leaf node; adding the target leaf node to the transformation rule; receiving a user's selection of at least one user-selected, optionally ambiguous, source leaf node, from a source tree of an initial source context, wherein the source leaf node represents a source XML element to be mapped to the target leaf node by the transformation rule; if the source leaf node is ambiguous, disambiguating the source leaf node; and adding the user-selected source leaf node to the transformation rule.