# The `--scale` command:

-R      [partitions:]<prior>                Prior on the Scale.
--scale [partitions:]<prior>

The branch lengths on the shared tree are multiplied by a
partition-specific scale factor to yield the branch lengths for
each partition. Estimating this scale factor allows estimating the
evolutionary rate for the partition.

If no prior is specified, then the default prior `~gamma(0.5,2)`
for the scale factor is used.

If no partitions are specified, as in

   --scale=~gamma(0.5,2)

then every partition gets a separate scale following the same
distribution.

If multiple partitions are specified in the same --scale command,
then they share not just the same distribution on the scale, but
also the same scale.  Thus,

   --scale=2,3:

indicates that partitions 2 and 3 share the same scale, which has
the default scale prior.

If the scale is specified as a constant, such as `--scale=1`,
then the scale will not be estimated.

The `--scale` command can be given multiple times, but the same
partition cannot be referenced twice.

# Examples:

   -R ~gamma(0.5,2)
   
   --scale=2,3:
   
   --scale=~gamma(0.5,2)
   
   --scale=1
