Singular Book 2.1.34 -- minimal presentations, prune

i1 : R = QQ[x,y,z,MonomialOrder=>{Weights=>{-1,-1,-1},RevLex},Global=>false]

o1 = R

o1 : PolynomialRing

i2 : M = cokernel matrix{{0,y},{x*y-1,x*z},{x*y+1,x*z}}

o2 = cokernel | 0     y  |
              | -1+xy xz |
              | 1+xy  xz |

                            3
o2 : R-module, quotient of R

i3 : prune M -- INCORRECT

o3 = cokernel | y  0     |
              | xz -1+xy |
              | xz 1+xy  |

                            3
o3 : R-module, quotient of R