COGENT Frequently Asked Questions

The purpose of this page is to answer some Frequently Asked Questions (FAQs) about COGENT, and provide some useful hints and tips for model construction. The page itself is under constant construction.

Questions fall under three main headings:

General Questions
Installation Questions
Technical Questions (of the "how do I do X?" variety)

There is also a form at the bottom of this page allowing you to submit a faq.

General Questions

Q: What does COGENT stand for?

A: Cognitive Objects within a Graphical EnviroNmenT

Q: Is there a Mac version of COGENT?

A: It is now possible to run COGENT on a Mac provided you are running Mac OS X. You will also need to install an X windows server, which is available at no cost from Apple's OS X software web site. [Note: The X in "OS X" and the X in "X Windows" refer to different things. OS X, and even a "server version of OS X" does not normally come with an X windows server.] If you have OS X and X windows, then please contact us (r.cooper@bbk.ac.uk) and we'll be happy to provide the executable.

Installation Questions

Q: The documentation for installing COGENT Version 2.1 on Windows suggests that COGENT should be installed in `C:\Program Files\COGENT`, but the installation procedure defaults to `C:\COGENT`. Which is correct?

A: There is an inconsistency between the installation procedure and the documentation for Version 2.1 (sorry!). We recommend using the standard configuration but installing in C:\Program Files\COGENT (rather than C:\COGENT as the installation routine would prefer). However, it really doesn't matter where you install COGENT -- it's a matter of taste.

Q: When I try to execute COGENT after installing it the following error is reported:

 
ld.so.1: cogent: fatal: libxview.so.3: open failed: No such file or directory
Killed

A: This error only occurs with the very old Unix version of COGENT, which uses the standard xview libraries, and which will only execute if these libraries are available. The environment variable LD_LIBRARY_PATH specifies the location of the libraries (normally /usr/openwin/lib). Find out where the libraries are located and set the environment variable (from the Unix prompt) accordingly. For example:

setenv LD_LIBRARY_PATH /usr/openwin/lib

COGENT should now work. Setting the environment variable should probably be automated by adding the above Unix shell command (or an appropriate variant of it) to COGENT users' .cshrc files.

Technical Questions

Q: What's the difference between refracted and unrefracted rules?

A: Refraction is a standard concept from the production system literature. It is often the case with non-triggered rules (typically rules which fire when certain elements are present in a buffer) that the rule should fire just once for each possible instantiation of its variables, and the rule should not fire on every cycle with the same variable binding. Refracted rules fire just once for any given variable binding.

Q: If an element is added to a propositional data store then it appears that all rules which match that particular element will fire only once. This seems to be true even if the element is added, then deleted then added again. How can I make a rule that fires once each time the element is added?

A: This behaviour occurs when a rule is marked as refracted (which is the default for non-triggered rules). If you want the rule to fire on every cycle, you'll need to set "Rule is refracted" off for the rule in question. If you want the rule to fire with one instantiation, then not fire until the element is deleted and added again, you need to do something a bit hacky: make sure that each time the element is added it is slightly different. This can be done by including the cycle number on which the element was added as an argument within the element:

BEFORE:
        Rule (refracted): Add an element to a buffer, thus triggering another rule
	IF:	some conditions
	THEN:	add term(arg) to buffer

        Rule (refracted): Triggered rule, that will only fire once per value of X
	IF:	term(X) is in buffer
	THEN:	some actions

AFTER:
        Rule (refracted): Add an element to a buffer, thus triggering another rule
	IF:	some conditions
		the current cycle is N
	THEN:	add term(arg, N) to buffer

        Rule (refracted): Triggered rule, that will only fire once per value of X
	IF:	term(X, N) is in buffer
	THEN:	some actions

Now, when the element is deleted and added again, it will not be exactly the same, allowing the refracted rule to fire again.

Q: I can't work out why my rule won't fire. What can I do?

A: You can use the "trace" qualifier on conditions of the rule. If they satisfy, the instantiation patterns will be printed out in the OOS window.

Q: Although COGENT buffers have configurable access properties, allowing first-in/first-out and last-in/first out access, I find that all items in a buffer are output on each cycle. How can I make COGENT buffers output only one item on each cycle, in the specified order?

A: To match only one item per cycle, use the "once" qualifier on a match condition. Then the condition will only satisfy once per cycle. Note that unless the buffer contents change before the next access, for example by deleting the first-matched item, the same item will be output each time.

Q: When I send a trigger to a process, it always takes effect on the next cycle. How can I delay execution of a trigger until some specified later time?

A: Define a rule with the trigger pattern "wait(X)", whose only action is to send the trigger X recurrently to the process. Then to delay the trigger "foo(bar)" by one cycle, use the construction "wait(foo(bar))". These can be nested, so for a delay of two cycles use "wait(wait(foo(bar)))" and so on.

Submit a FAQ

If you have a question that isn't answered above but which you think might qualify as a FAQ, please use the form below to submit the question. We'll then get back to as soon as possible.