| brink | Small taper dataset from Brink and von Gadow (1986). | 
| decay | Decay function delta | 
| hlevel | Height level for given diameter or area. | 
| Id | Integral of decay function. | 
| Idd | Double integral of decay function. | 
| taper | Tree taper (or profile) equation. | 
| tbase | Unscaled base taper curve of cross-sectional area vs height level. | 
| volume | Volume between two height levels. |