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