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2.1 Elements which Determine the Flow of Control

Most code which is written using Aspen SCM Expert System is procedural. In order to understand existing code and be able to modify it or write new code, you need to understand how rules are executed.

The elements of Aspen SCM Expert System which determine the flow of control through rules are:

  • language elements:
    • the keywords IF and THEN in the Code of a rule;

    • the keywords AND and OR in the Code and in the Description of a rule;

    • the brackets ( and ) in the Description of a rule;

    • the keyword NOTE in the Code of a rule and in-line comment delimiters /* and */ in the Description of a rule;

    • the looping constructs WHILE and IN;
  • the mechanism of predicate assertion and passing of arguments to rules;

  • parameters in the table CNTLE, specifically:

    • CHAIN: forward or backward chaining;

    • LOOP: iteration limit for WHILE loops;

    • EOU: whether the interpreter stops when a statement resolves UNKNOWN;

    • UNIF: predicate unification flag.

Underlying everything is:

  • the interpreter’s mechanism for evaluating rules to establish whether they are true or not.


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