XML documents are increasingly being used to mark up various kinds of data from web content to scientific data. Often these documents need to be collaboratively created and edited by a group of users. In this paper we present a flexible solution for supporting collaboration over XML documents. Merging of user work is based on the operations performed. A key to achieving flexibility for the definition and resolution of conflicts was to keep the operations distributed throughout the tree model of the document associated with the nodes to which they refer.