Abstract: Methods, systems, and apparatus, including computer programs encoded on a computer storage medium, for storing data from a hierarchical structure with labels that encode the data's respective position within a data structure that maps hierarchically structured information into relationally structured data. The data structure includes physical nodes, where each physical node represents a data node of the hierarchical structure, and virtual nodes, where each virtual node represents a type of hierarchical relationship between corresponding physical nodes. Each virtual node serves as an expansion node that permits addition and deletion of data within the hierarchical structure without altering labels associated with existing data.