Sigma is an open source environment for the
development of logical theories. It has been under development and regular release for nearly a decade, and has been the principal environment under which the open source Suggested Upper Merged Ontology (SUMO) has been created. We discuss
its features and evolution, and explain why it is an appropriate environment for the development of expressive ontologies in first and higher order logic.
Sigma: An Integrated Development Environment for Formal Ontology