def **(x)
      check_operable
      m = Utils.as_numeric(x)
      dims = dimension_uop{|a| a*m}
      Unit.new(@factor**m,dims)
    end
def **(x)
      check_operable
      m = Utils.as_numeric(x)
      dims = dimension_uop{|a| a*m}
      Unit.new(@factor**m,dims)
    end
Exponentiation of units.
 This units must be operable.
 @param  [Numeric] x  numeric
 @return [Phys::Unit]
 @raise  [Phys::UnitError] if not operable.