The Visual Logic Tool

Visual Logic is a software tool designed to explore the logical concepts presented in the preceding discussion. It provides various means for the visual display and manipulation of the associated clause trees. The tool is implemented using XPCE, an object-oriented graphical interface package for SWI-Prolog (University of Amsterdam) and other symbolic languages. XPCE/Swi-Prolog is available on various flavors of Unix and X-Windows as well as Microsoft Windows 3.x/95/NT platform. (It is being developed on Windows and will probably be ported to Unix once a license is obtained.)

The tool is composed of several components:

• main module: integrates the various modules and provides the main user interface

• computation module: computes derivations for the system.

• Explore module: lets the user explore the logic program's space in a non-linear fashion.

• Query module: lets the user query the system with specific goals, such as, "show all supported propositions" and provides an 'explanation' in the form of annotated clause trees.

• Results module: accumlates and shows all known derivations.

• a built-in editor: lets the user edit the input source file (i.e., the program clauses).

Below is a screen shot of the tool. Due to limitted screen space, not all windows are fully visible.

Each of the graphical windows corresponds roughly to a module. The main window (at top) is the command center for the tool. Here, the user can load the source file, quickly switch between the various windows, launch the built-in editor to modify the source file, and quit the tool. The other windows will be described in details later. For now, note that the Query window shows an annotated clause tree for a (see the example in previous page). Contingency sets are shown in list notation, with brackets in place of curly braces. The nodes are color coded as followed:
• Blue: Evident.
• Purple (large font): Supported.
• Bright Red: Conflicted.
• Bright Red (large font): Conflicted and supported.
• Dark Red: Node cannot be expanded, either because there is no matching clause or a cycle is detected.
• Green: either TRUE or ancestry resolved (a node whose ancestor is its negation).
• Black: None of the above.

The following logic program dealing with a (simplified) trial will be used in the subsequent discussion. The program is borrowed from John Fisher.

guilty <- motive, opportunity, forensic_evidence.
motive <- jealous, angry.    
motive <- possessive, angry.

~guilty <- alibi.
alibi <- seen_elsewhere.     
alibi <- incapacitated.

opportunity <- nearby, unobserved.

forensic_evidence <- dna_tests, blood_match.
~forensic_evidence <- framed.

framed <- planted_glove.
framed <- faked_tests.

~unobserved <- seen_elsewhere.

jealous <- true.             
angry <- true.     
possessive <- true.
nearby <- true.
unobserved <- true.
dna_tests <- true.
blood_match <- true.
planted_glove <- true.