Forward and Backward Chaining

Let us consider an example consisting of four rules made up of several conditions (i.e server status, user type, user name) and actions (i.e., deny access, grant access, connect to server)  

Suppose we need to check if the server is connected, given the server is online and Jill tries to log on. 

In forward-chaining technique, a rule that applies is searched for in the first iteration. Rule 3 applies, and the user type is defined as Administrator. In the second iteration rule 2 applies and access is granted, which finally leads to application of rule 4 in the third iteration. 

In backward chaining the rules are evaluated starting from the conclusion (i.e., server is connected) and finding out the necessary condition. In first iteration the access must be granted (rule 4) to connect to the server. To grant access, the server must be online — one of the given assertions — and the user must be an administrator (rule 2). Since the online criterion is one of the given assertions, rule 3 holds the condition to be fulfilled. 

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The system s inference engine offers one of the largest sets of reasoning techniques. Besides forward and backward chaining, examples are as follows ... 

Facts, rules and quantitative models are the chosen forms of representation for the different areas of knowledge in the field of wool dyeing. But these parts are not independent, they must be tied together, i.e. an inference mechanism is needed. The theoretical concepts of forward and backward chaining in their pure forms turned out to be insufficient to catch fully the expert s knowledge about the connections and interdependencies between the various aspects. Only a careful detailed analysis of the expert s behavior allowed implementation of an adequate inference mechanism, which turned out to be a mixture of several theoretical approaches. 

Three hazard analysis techniques are currently used widely Fault Tree Analysis, Event Tree Analysis, and HAZOP. Variants that combine aspects of these three techniques, such as Cause-Consequence Analysis (combining top-down fault trees and forward analysis Event Trees) and Bowtie Analysis (combining forward and backward chaining techniques) are also sometimes used. Safeware and other basic textbooks contain more information about these techniques for those unfamiliar with them. FMEA (Failure Modes and Effects Analysis) is sometimes used as a hazard analysis technique, but it is a bottom-up reliability analysis technique and has very limited applicability for safety analysis. 

This process of forward and backward chaining is continued until all of the hierarchy of buyer objectives have been evaluated. Although for each objective, every robot that meets the criteria is recorded, only the ones that meet all of the objectives are considered for the choice of best robot. At the end ofthe querry the list of best robot is given in report form. The following is an example of the type of logic rule construction used. 

Whether it follows forward or backward chaining, the system will pose a series of questions that allow it to gather more information about the problem until it successfully answers the user s query or runs out of alternatives and admits defeat. Just like the way that the system chooses to respond to a query, the exact sequence of steps that it takes is not preprogrammed into the knowledge base, but is determined "on the fly" by the scheduler, which reassesses the options after each response that the user provides. 

FIGURE 1.24. Potential-dependent forward and backward rate constants of the ferrocene-ferrocenium couple attached to a gold electrode hy a long-chain alkane thiol assembled together with unsubstituted alkane thiols of similar length. Solid line use of Equations (1.37) to (1.39) with X, = 0.85 eV, ks — 1.25 s 1. Adapted from Figure 4A in reference 65, with permission from the American Association for the Advancement of Science. 

Inference paradigms. The basic inference mechanisms of forward-chaining and backward-chaining must be integrated into a real-time execution environment. 

Fig. 3.13. Diagrammatic representation of the Forward-Backward procedure. A double-stranded DNA fragment [32P] labelled (asterisk) at one 5 -end is represented at the top of the figure. DNA polymerase I and a nucleotide chain inhibitor (e.g. ddA) are added. Contaminating DNAases in the Poll preparation produce nicks, indicated by the vertical arrows. From the 3 -end created by each nick, the reaction catalysed by Poll proceeds in the 5 - to 3 -direction (Forward reaction) provided dNTPs (dG, dT, dC) are present if they are not added the reaction proceeds exonucleolytically in the 3 - to 5 -direction (Backwards). The numbered lines represent the DNA fragments which arise from the similarly numbered DNA nicks. The hypothetical DNA sequence illustrates the complementary results obtained from the Forward and Backward reactions with repeated nucleotides e.g. the sequence AA. In the Forward reaction the proximal A will be represented by a strong band and the distal A by a weak band. The converse is true for the Backward reaction. The dotted lines 4 and 5 signify those reactions which proceed 5 - 3 (Forward) in the Backwards procedure.

Enzyme-catalyzed reactions, including the electron transport chain and proton translocation, are composed of series of elementary reactions that proceed forward and backward. One of the methods in describing this thermodynamically and mathematically coupled complex chemical reaction-transport system is the nonequilibrium thermodynamic model, which does not require the detailed knowledge of the system. 

It has been shown by Doi [11] that on the time scale of Tg contour length fluctuations may induce a rapid relaxation of chain ends, especially for moderately long chains. Indeed, wiggling motions involve forward and backward motions of the chain ends. Thus, chain length fluctuations in oriented materials lead to the creation of isotropic parts of tubes at each end. Their fractional length is roughly equal to 1.3(N e) > where N is the number of monomers per chain. An elaborate expression for the relaxation due to this mechanism, based on the Pearson-Helfand picture for star pol5uners [12], has been proposed by Viovy [13]. 

Reversible reactions are those in which two substances entering a single simple consecutive chain reaction interact in both forward and backward directions, i.e. ... 

If no stationary distribution Psi(x) exists or if it is unknown, a different approach must be taken. For this purpose, we return to Eq. (1.56) and insert the forward and backward generation probabilities from Eqs. (1.61) and (1.62). Using Eq. (1.32) and applying the chain rule for products of Jacobians, we finally obtain the following expression for the acceptance probability of a forward shifting move. 

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