def sub_step(value, type = :next)
      val = value.abs
      type = :prev if value < 0
      command(type == :next ? "sub_step #{val}" : "sub_step #{-val}" )
    end
def sub_step(value, type = :next)
      val = value.abs
      type = :prev if value < 0
      command(type == :next ? "sub_step #{val}" : "sub_step #{-val}" )
    end
Step forward in the subtitle list by <value> steps
 step backwards if <value> is negative
 can also set type to :backward or :forward and return postive <value>