| #### D.2.4.6 Prep Procedure from library `grobcov.lib` (see grobcov_lib). Usage: Prep(ideal N,ideal M); ideal N (null ideal) (not necessarily radical nor maximal) in Q[a]. (a=parameters, x=variables). ideal M (hole ideal) (not necessarily containing N) in Q[a]. To be called in a ring Q[a][x] or a ring Q[a]. But the ideals can contain only the parameters in Q[a]. Return: The canonical P-representation of the locally closed set V(N) \ V(M) Output: [Comp_1, .. , Comp_s ] where Comp_i=[p_i,[p_i1,..,p_is_i]] Example:
    LIB "grobcov.lib";
    short=0;
    if(defined(R)){kill R;}
    ring R=0,(a,b,c),lp;
    ideal p=a*b;;
    ideal q=a,b-1;
    // P-representation of V(p) \ V(q)
    Prep(p,q);
    ==> [1]:
    ==>    [1]:
    ==>       _[1]=b
    ==>    [2]:
    ==>       [1]:
    ==>          _[1]=1
    ==> [2]:
    ==>    [1]:
    ==>       _[1]=a
    ==>    [2]:
    ==>       [1]:
    ==>          _[1]=b-1
    ==>          _[2]=a

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