Abstract: A method of accessing a database using a computer system. One example of the method includes the following steps. Access a tree data structure. The tree data structure includes a set of nodes linked in parent, child and sibling relations. Each node of a subset of nodes is associated with a corresponding term. Select some nodes of the set of nodes. This includes selecting a node of the set of nodes. The node has at least a sibling node. In response to selecting the node, performing one of the following actions. If the node is a first type of node, allowing a selection of the sibling node. If the node is a second type of node, not allowing a selection of the sibling node. Record a list of node selections corresponding to the selection of some nodes. Extract a set of terms from the list of node selections. Create a data structure including price information and duration of existence information. Store the data structure in the database using the set of terms.