Description of problem:
I wanted to install f18 like this:
/boot btrfs /dev/sda1 /dev/sdb6 raid1
/        btrfs /dev/sda2 /dev/sdb7 raid1

But the installer does not let me do it.
1} If I create the fs myself prior to installation, then the mount point for / is not selected as it requires
     reformat but reformat option is greyed, so it won't let me install.
2} If I create the partitions but do not format them prior to installation then the options for mount points
     & formatting etc remain greyed & cannot be selected. Clicking + sign asks for desired size which I interpret
     as creating a new partition. Basically one cannot select a prior partition table for installation if it is to be btrfs.
3} Also one cannot have boot & root or any other mount point on different partitions if one goes by creating mount
     points from the GUI for btrfs. The logic just bunches up togather all btrfs mounts on single volume - here- partition
4} If a partition does not have recognizable fs then it is not possible to select btrfs as fs from filesystems in reformat.

So What I did was that I created btrfs on /dev/sda1 as label boot & /dev/sda2 as label root prior to installation.
Booted the install disk. Selected only one disk. Selected BTRFS & manual partition.

I could now select sda1 for boot & also could click reformat as btrfs. So I selected the partition & selected mount
point /boot & reformat as btrfs

Same I did with the second partition for /

Started installation & it crashed
The following was filed automatically by anaconda:
anaconda 18.37.11 exception report
Traceback (most recent call first):
  File "/usr/lib64/python2.7/site-packages/pyanaconda/storage/formats/fs.py", line 594, in mount
    raise FSError("filesystem has not been created")
  File "/usr/lib64/python2.7/site-packages/pyanaconda/storage/formats/fs.py", line 1140, in setup
    return self.mount(*args, **kwargs)
  File "/usr/lib64/python2.7/site-packages/pyanaconda/storage/__init__.py", line 2881, in mountFilesystems
    chroot=rootPath)
  File "/usr/lib64/python2.7/site-packages/pyanaconda/storage/__init__.py", line 1655, in mountFilesystems
    readOnly=readOnly, skipRoot=skipRoot)
  File "/usr/lib64/python2.7/site-packages/pyanaconda/storage/__init__.py", line 193, in turnOnFilesystems
    skipRoot=False)
  File "/usr/lib64/python2.7/site-packages/pyanaconda/install.py", line 114, in doInstall
    turnOnFilesystems(storage)
  File "/usr/lib64/python2.7/threading.py", line 504, in run
    self.__target(*self.__args, **self.__kwargs)
  File "/usr/lib64/python2.7/site-packages/pyanaconda/threads.py", line 91, in run
    threading.Thread.run(self, *args, **kwargs)
FSError: filesystem has not been created

Additional info:
cmdline:        /usr/bin/python  /sbin/anaconda
cmdline_file:   initrd=initrd.img inst.stage2=hd:LABEL=Fedora\x2018\x20x86_64 quiet BOOT_IMAGE=vmlinuz 
executable:     /sbin/anaconda
hashmarkername: anaconda
kernel:         3.6.10-4.fc18.x86_64
other involved packages: 
package:        anaconda-18.37.11
product:        Fedora
release:        Cannot get release name.
type:           libreport
version:        18