We used the term relation to represent our generalisation of the stream concept. A relation is a data structure which is used to organize linguistic items into linguistic structures such as trees and lists. A relation is a set of named links connecting a set of nodes. Lists (streams in the MLDS) are linear relations where each node has a previous and next link. Nodes in trees have up and down links also. Nodes are purely positional units, and contain no information apart from their links. In addition to having links to other nodes, each node has a single link to an item, which contains the linguistic information.
Figure 1 shows an example utterance with a syntax relation and a word relation. The word relation contains nodes which have next and previous connections, whereas the syntax relation, has up and down connections in addition to next and previous. Each node in the syntax tree is linked to a item, each of which has a feature, CAT, giving its syntactic category. Terminal nodes in a syntax tree are words, and so an additional feature name is used here. The nodes in the word relation, which is a linear list, are also linked to the items that are linked to the terminal nodes in the syntax tree.
In this way, node structures of arbitrary complexity can be constructed, and they can be intertwined in a natural way by having links from different nodes to the same item.