Calculates the staircase-ness measure.

1 | ```
stairs(tree)
``` |

`tree` |
a tree of class |

Two numbers corresponding to the two staircase-ness measures for a tree. Defined in Norstrom et al., Evolutionary Bioinformatics online, 8:261 (2012) doi: 10.4137/EBO.S9738, these are two related measures:

1: the proportion of subtrees that are imbalanced (i.e. subtrees where the left child has more tip descendants than the right child, or vice versa)

2: the average of all the min(l,r)/max(l,r) values of each subtree, where l and r are the number of tips in the left and right children of a subtree.

Michael Boyd mboyd855@gmail.com

Michelle Kendall michelle.louise.kendall@gmail.com

1 2 3 | ```
## Find the staircase-ness measures in a random tree with 20 tips:
stairs(rtree(20))
``` |

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