Bug in diff and inverting variable in rings over a fraction field

[mahrud]

i1 : R = frac(QQ[x,y])[z];

i2 : 1/x

     1
o2 = -
     x

o2 : frac(QQ[x..y])

i3 : 1/x_R
stdio:3:2:(3): error: not implemented yet: fraction fields of polynomial rings over rings other than ZZ, QQ, or a finite field

i4 : x_R^-1

     1
o4 = -
     x

o4 : R