names attribute does not survive arithmetic operations

[RobinHankin]

> x <- rfree(9,2)
> names(x) <- letters[1:9]
> x
        a         b         c         d         e         f         g         h 
        0       b^4      a^-2 a^-1.b^-2     a.b^2      b^-1     a^2.b      b^-2 
        i 
     b^-3 
> x+x
[1] 0                   b^8                 a^-4               
[4] a^-1.b^-2.a^-1.b^-2 a.b^2.a.b^2         b^-2               
[7] a^2.b.a^2.b         b^-4                b^-6               
>