# The `--branch-lengths` command:

-B               <prior>                    Prior on branch-lengths.
--branch-lengths <prior>

Specifies the prior for branch lengths of the common tree, before
partition-specific scale factors are applied. The prior should be
a list with an entry for each branch of the tree.  The tree can
be referenced as the variable T.

The default branch-length prior is:

   ~iid[num\_branches[T],gamma[0.5,div[2,num\_branches[T]]]]

Since the expected total tree length is 1 under this prior, the
expected total tree length for each partition is given by the
scale factor.

# Examples:

   -B ~iid[num\_branches[T],gamma[0.5,div[2,num\_branches[T]]]]
   
   --branch-length iid[num\_branches[T],~exponential[div[1,num\_branches[T]]]]
