A significant recent addition to the repertoire of grammar-based approaches to genetic programming (GP) is the use of tree-adjunct and tree-adjoining grammars (TAG). A TAG is a tree-generating grammar, and as such is a natural representation for GP and for computer programs. One of the primary benefits of a TAG is that the application of any rule within a TAG to an existing tree results in an executable tree, which overcomes the possibility of creating invalid programs (incomplete programs which cannot be executed). The creation of invalid phenotypes can occur in methods such as grammatical evolution without enforcing some sort of repair-like strategy during the genotype–phenotype mapping process.