`N1Spr()`

calculates the number of trees one subtree prune-and-regraft
operation away from a binary input tree using the formula given by
Allen and Steel (2001)
;
`IC1Spr()`

calculates the information content of trees at this
distance: i.e. the entropy corresponding to the proportion of all possible
*n*-tip trees whose SPR distance is at most one from a specified tree.

## Value

`N1Spr()`

returns an integer vector denoting the number of trees one
SPR rearrangement away from the input tree..

`IC1Spr()`

returns an numeric vector giving the phylogenetic
information content of trees 0 or 1 SPR rearrangement from an *n*-leaf tree,
in bits.

## References

Allen BL, Steel MA (2001).
“Subtree transfer operations and their induced metrics on evolutionary trees.”
*Annals of Combinatorics*, **5**(1), 1–15.
doi:10.1007/s00026-001-8006-8
.