| angle | Angle between two subspaces | 
| hier.phd.nt | Main hierarchical SDR fitting function | 
| hier.sphd | Main hierarchical sufficient dimension reduction fitting function | 
| phd | PHD SDR fitting function | 
| plot.hier_sdr_fit | Plotting hierarchical SDR models | 
| projnorm | Norm of difference of projections | 
| semi.phd | Semiparametric PHD SDR fitting function | 
| simulate_data | Simulate data with hierarchical subspaces |