Initial release

Initial release of Nengo SPA with core functionality, but excluding

• updates and completion the documentation,
• proper integration with Nengo GUI.

The API is still conisdered unstable in some parts of it are likely to change in the future.

Main features compared to the SPA implementation shipped with Nengo are:

• neural representations have been optimized for higher accuracy,
• support for arbitrarily complex action rules,
• SPA networks can be used as normal Nengo networks,
• and SPA networks can be nested.