be aware of and respond to this update, thus maintaining the consistency of the relationship between values in the program. An example of a reactive language that provides such a cell is FrTime. Properly encapsulated object-oriented libraries offer an encapsulated notion of state. In principle, it is therefore possible for such a library to interact smoothly with the reactive portion of a language. For instance, callbacks can be installed in the getters of the object-oriented library to notify the reactive update engine about state changes, and changes in the reactive component can be pushed to the object-oriented library through getters. FrTime employs such a strategy. Dynamic updating of the graph of dependencies In some reactive languages, the graph of dependencies is static, i.e., the graph is fixed throughout the program's execution. In other languages, the graph can be dynamic, i.e., it can change as the program executes. For a simple example, consider this illustrative example (where seconds is a reactive value): t = if ((seconds mod 2) == 0): seconds + 1 else: seconds - 1 end t + 1 Every second, the value of this expression changes to a different reactive expression, which t + 1 then depends on. Therefore, the graph of dependencies updates every second. Permitting dynamic updating of dependencies provides significant expressive power (for instance, dynamic dependencies routinely occur in graphical user interface (GUI) programs). However, the reactive update engine must decide whether to reconstruct expressions each time, or to keep an expression's node constructed but inactive; in the latter case, ensure that they do not participate in the computation when they are not supposed to be active. Concepts Degrees of explicitness Reactive programming languages can range from very explicit ones where data flows are set up by using arrows, to implicit where the data flows are derived from language constructs that look similar to those of imperative or functional programming. For example, in implicitly lifted functional reactive programming (FRP) a function call might implicitly cause a node in a data flow graph to be constructed. Reactive programming libraries for dynamic languages (such as the Lisp "Cells" and Python "Trellis" libraries) can construct a dependency graph from runtime analysis of the values read during a function's execution, allowing data flow specifications to be both implicit and dynamic. Sometimes the term reactive programming refers to the architectural level of software engineering, where individual nodes in the data