Functions to describe the position of a leaf relative to the root. 'Stemmier' leaves ought to exhibit a smaller root-node distance and a larger sister size,

```
SisterSize(tree, tip)
# S3 method for numeric
SisterSize(tree, tip)
# S3 method for character
SisterSize(tree, tip)
RootNodeDistance(tree, tip)
# S3 method for numeric
RootNodeDistance(tree, tip)
# S3 method for character
RootNodeDistance(tree, tip)
RootNodeDist(tree, tip)
```

- tree
A tree of class

`phylo`

.- tip
Either a numeric specifying the index of a single tip, or a character specifying its label.

`SisterSize()`

returns an integer specifying the number of leaves
in the clade that is sister to `tip`

.
`RootNodeDist()`

returns an integer specifying the number of nodes between
`tip`

and the root node of `tree`

.

`RootNodeDistance()`

calculates the number of nodes between the chosen leaf
and the root of `tree`

.
This is an unsatisfactory measure, as the range of possible
distances is a function of the shape of the tree.
As an example, leaf *X1* in the tree `(.,(.,(.,(.,(X1,(a,b))))))`

falls outside the clade *(a, b)* and has a root-node distance of 4,
whereas leaf *X2* in the tree `(.,((.,(.,.)),(b,(X2,a))))`

falls within the clade *(a, b)*, so should be considered more 'crownwards',
yet has a smaller root-node distance (3).

`SisterSize()`

measures the number of leaves in the clade that is sister to
the chosen leaf. In the examples above, *X1* has a sister size of 2 leaves,
whereas *X2*, which is 'more crownwards', has a smaller sister size (1 leaf),
as desired.

Asher R, Smith MR (2021).
“Phylogenetic signal and bias in paleontology.”
*Systematic Biology*, **Online ahead of print**, syab072.
doi: 10.1093/sysbio/syab072
.

Other tree characterization functions:
`CladisticInfo()`

,
`Consensus()`

,
`TotalCopheneticIndex()`

```
bal8 <- BalancedTree(8)
pec8 <- PectinateTree(8)
SisterSize(bal8, 3)
#> [1] 1
SisterSize(pec8, 't3')
#> [1] 5
SisterSize(RootTree(pec8, 't3'), 't3')
#> [1] 7
RootNodeDist(bal8, 3)
#> [1] 2
RootNodeDist(pec8, 't3')
#> [1] 2
RootNodeDist(RootTree(pec8, 't3'), 't3')
#> [1] 0
```