def -(other)
      if other.is_a? Numeric
        tags[:size_mod] = -other
        self
      else
        self_equiv, arg_equiv = other.coerce(self)
        self_equiv - arg_equiv
      end
    end
def -(other)
      if other.is_a? Numeric
        tags[:size_mod] = -other
        self
      else
        self_equiv, arg_equiv = other.coerce(self)
        self_equiv - arg_equiv
      end
    end
Modifies +self+ such that it sizes itself (on {#to_s}ing), and
 keeping this size modification in mind.  In other words, this
 is meant to be used in another directive's {#[]} method for
 telling it what size it should be, and this method modifies that
 size by the given amount.

 @example
   some_directive[another_directive - 5]
 @param other [Numeric, #coerce]
 @return [self, Object]