Incorrect kernel computation

there's definitely something wrong here: (kernel should be generated by x-y)
```
i1 : F=frac(QQ[x])

o1 = F

o1 : FractionField

i2 : R1=F[]

o2 = R1

o2 : PolynomialRing

i3 : R2=F[y]

o3 = R2

o3 : PolynomialRing

i4 : f=map(R1,R2,{x})

o4 = map (R1, R2, {x, x})

o4 : RingMap R1 <--- R2

i5 : kernel f

o5 = ideal ()

o5 : Ideal of R2

i6 : f (x-y)

o6 = 0

o6 : R1
```

Reproduce the transcript in a Macaulay2 session, beginning with the construction of R1, R2, and the map f, then inspect the kernel computation for f. The issue is resolved when kernel f includes the demonstrated relation x-y rather than returning the empty ideal.

Written by the indexing model from the issue text.