The Sigma knowledge engineering environment is an system for developing, viewing
and debugging theories in first order logic. It works with Knowledge
Interchange Format (KIF) and is optimized for the
Suggested Upper Merged
Sigma includes a number of useful features for knowledge engineering work, including term and hierarchy browsing, the ability to load different files of logical theories, a full first order inference capability with structured proof results, a natural language paraphrase capability for logical axioms, support for displaying mappings to the WordNet lexicon and a number of knowledge base diagnostics.
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Main SourceForge page for Sigma