sentences:  Such static-semantic constraints may be specified imperatively by actions (e.g., as in Gandalf), or declaratively by an attribute grammar (e.g., as in the Synthesizer Generator) or by unification in a many-sorted algebra (e.g., as in PSG ) or a logic program (e.g., as in Centaur and Pan), with compliance checked by the underlying editing machinery.Structured editors vary in the degree to which they allow their users to perform edits that cause the document to become syntactically or semantically incorrect.It is common for a language sensitive editor to represent a document as a parse tree with respect to language’s grammar, or as an abstract syntax tree (AST).