RobustGeometry.NET is published under the MIT license.
The original C code for the robust geometric predicates was placed in the public domain by Jonathan Richard Shewchuk.
The elegant .NET generics design for the halfedge data structure is described here by Alexander Kolliopoulos and implemented in his Lydos library, also published under the zlib license.
The MIT License (MIT)
Copyright (c) 2012-2014 Govert van Drimmelen
Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:
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