implement kernel of ringmap over ZZ

    i1 : R = ZZ[x]

    o1 = R

    o1 : PolynomialRing

    i2 : f = map(R,R,{0})

    o2 = map(R,R,{0})

    o2 : RingMap R <--- R

    i3 : ker f
    stdio:3:1:(1):[0]: kernel Ringmap: not implemented yet
