def coerce(object)
      if object.kind_of? Numeric
        return Quantity.new(object, Unit.unity), self
      else
        raise Exceptions::InvalidArgumentError, "Cannot coerce #{self.class} into #{object.class}"
      end
    end
def coerce(object)
      if object.kind_of? Numeric
        return Quantity.new(object, Unit.unity), self
      else
        raise Exceptions::InvalidArgumentError, "Cannot coerce #{self.class} into #{object.class}"
      end
    end
Enables shorthand for reciprocal of quantity, e.g.

   quantity = 2.m

   (1/quantity).to_s :name                 #=> "0.5 per metre"