1. A computer-programming tool for generating an implementation of a first data structure, the first data structure representing at least a portion of a computer-programming language, the language comprising a plurality of syntactical elements satisfying a set of syntax rules and the first data structure comprising a plurality of linked nodes, such nodes comprising a root node, a number of first-tier nodes linked directly to the root node, and a number of subsequent-tier nodes linked indirectly to the root node via one or more other said nodes, the nodes representing such syntactical elements of the language and a pattern of links between the nodes representing paths of inheritance of substitutability but not implementation and interface between those nodes, the tool comprising: memory operable to store one of said first data structure and a description thereof; and at least one processor configured to generate, from one said first data structure: one of a second data structure and a description thereof, corresponding to said first data structure, the second data structure comprising nodes corresponding to the nodes of the first data structure with all nodes of the second data structure other than its root node being linked directly to the root node of the second data structure, wherein the links between the nodes of the second data structure are representative of paths of inheritance of substitutability, implementation and interface between those nodes, the processor being further operable to generate, based on the pattern of links of said one said first data structure, implementation rules which define the rules of substitutability of the first data structure to be enforced in relation to nodes of the second data structure during a subsequent processing operation which utilizes said second data structure in order to establish compliance with the inheritance of substitutability represented by the first data structure, wherein the second data structure and the implementation rules form one said implementation of the first data structure.