Where a generative grammar attempts to list all the rules that result in all well-formed sentences, constraint-based grammars allow anything that is not otherwise constrained. Certain versions of dependency grammar, head-driven phrase structure grammar, lexical functional grammar, categorial grammar, relational grammar, link grammar, and tree-adjoining grammar are constraint-based grammars that have been proposed. In stochastic grammar, grammatical correctness is taken as a probabilistic variable, rather than a discrete (yes or no) property.