# File lib/eymiha/math3/quaternion.rb, line 131
    def subtract!(axis=origin,real=0)
      if axis.kind_of? Quaternion
        subtract!(axis.axis,axis.real)
      elsif axis.point3_like?
        set(@axis-axis,@real-real)
      else
        raise_no_conversion axis
      end
      self
    end