This visitor traverses an Expression and retrieves the "literal" value. The literal to retrieve depends on the type. The literal is basically a string value like an identification variable name, an input parameter, a path expression, an abstract schema name, etc.

Provisional API: This interface is part of an interim API that is still under development and expected to change significantly before reaching stability. It is available at this early stage to solicit feedback from pioneering adopters on the understanding that any code that uses this API will almost certainly be broken (repeatedly) as the API evolves.