PROLOG Rules

A PROLOG rule is an if-then construction in PROLOG notation this would look like the following  

Here X is a variable, where the notation requires an uppercase character in PROLOG — variables are not similar to those in conventional programming languages. 

If a set of facts and rules have been defined, a PROLOG program can be executed, which leads to a question-and-answer scenario. PROLOG tries to interpret the facts and rules to answers with either yes, no, or a predefined answer. For example, the following could be asked  

Compound = benzene Compound = toluene Compound = xylene 

Compound = benzene CompoundType = aromatic Compound = toluene 

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PROLOG Rules are conditional statements in PROLOG used in the inference process in the form Then-Statement(X) - if-conditionl(JO. if-condition2(J0, ... 

D. Translation of Grammar Rules into Prolog Rules... 

The remaining cases reclassified by Prolog and Helmchen are tetrahedral centers with four distinct ligands, of which two and only two are enantiomorphic ligands in diastereotopic positions (as in 16a, Fig. S). These centers were originally regarded as pseudoasymmetric (39), and had received descriptors that did not change on reflection. Therefore, no anomaly had to be corrected. When these centers were reclassified as chiral (5), the Sequence Rule was also modified ... 

The general grammars and the mixture of declarative and procedural Prolog code allows easy grammar rule writing for other reactions. As an additional example, consider heterocyclic rings. The grammar rule... 

A predicate called wln-order occurs within the makejsynthon predicate. This predicate builds a graph from the pseudo-WLN (using WLN Rule 8(a)) and possibly reorders the graph as described below. The following Prolog code describes this manipulation ... 

The next entry in the hrf list is hrf(acetanilide,(tb,35)), which is matched to hrf(A,T,X), resulting in A = acetanilide, T = tb and X = 35. The second condition then becomes hrf(B,(tb,Y)). Beginning again at the start of the list of hrf entries in the data base, the first match is with hrf(aconitine,(tb,45)), which sets B to aconitine and Y to 35. The third condition is satisfied with 45 not equal 35, and PROLOG has found another solution. From the chemist s point of view this solution is the same as the first one. However, from the point of view of PROLOG, the two solutions are distinct, as we did not supply PROLOG with an appropriate rule. Further backtracking does not produce other solutions. 

Prolog allows for replacing an old version of a rule by a new one. If instead of calling consult we call reconsult, any new rule typed in will erase all old rules with the same head. Thus to update the system we call reconsult(user) and type in the new hrf rule. The program behaves exactly as before, but the basic data compilation has become more compact and will be much easier to maintain and update. 

To implement such a rule we need to do some arithmetics to determine the position of the limits of the tolerance window. PROLOG is not an arithmetic language. Thus handling of numerical calculations is somewhat clumsy. The standard arithmetic operators are part of the language. However, they are interpreted as specifying a relation between numbers rather than as commands to perform a calculation. In particular, the operator = signifies equality, but does not trigger the evaluation of an arithmetic expression. 

With these peculiarities of PROLOG in mind, we-formulate our rfmatch rule as follows ... 

In the hrf predicate a test for equality of Rf values is made implicitly by the PROLOG system. This makes modification more difficult. In order to replace the equality test with the new version we need first to move the respective test from the system level to the rule level. Thus, we retrieve from the data base the theoretical value Rfth which then is compared to the specified value given as argument. So we add the following new version ... 

Rank the following sots of substituents in order of priority according to the Cahn-Ingold—Prolog sequence rules ... 

A PROLOG program consists of a database including facts and rules, which are evaluated systematically when a query is defined by a user. The result of this evaluation can be either positive or negative. The outcome is positive if a logical deduction can be made otherwise it is negative if the current database does not allow a deduction. 

A PROLOG program can be asked whether a fact is true or whether a logical statement is consistent with the rules stored in the program. These questions can be answered independently of a concrete hard-coded code. 

PROLOG systematically goes through all its facts and rules and tries to find all the ways it can associate variables with particular values so that the initial query is satisfied. 

PROLOG matches aromaticCompound(A) against the head of the rule aromaticCompound(X). The first condition in the rule coplanar(X) can be satisfied with cyclopentadiene that is, cyclopentadiene is bound to A. The next condition cyclicConjugated(X) does not apply, so PROLOG goes back to the first condition, where it now binds benzene to A. PROLOG checks all facts that match the rule (the three conditions) until it finds an answer the only fact matching all three rules for aromaticity is benzene. 

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