Half of a 120-cell

This model is featured in figure 3.22 of [Visualizing Mathematics with 3D Printing](http://3dprintmath.com).

This is joint work with Saul Schleimer.

The 120-cell is one of the six regular 4-dimensional polytopes. This is the result of taking the edges of the polytope, radially projecting them onto the unit 3-sphere in 4-dimensional space, then stereographically projecting the result into our 3-dimensional space. We remove the half of the design in the hemi-3-sphere nearest to the projecti