# if a user has been specified, first get their UID (from their
  # username if necessary).

  ...

  # now divine the user's permissions.  first get the file's mode
  # bits and ownership

  ...

  # now check user/group perms.  Set isReadable etc if the mode has
  # the owner bit set and the user is the owner, or has the group bit
  # set and the user is in the right group

  ...

  # root can read and write anything, can execute anything
  # with any x bit set