| #### D.2.4.5 Crep Procedure from library `grobcov.lib` (see grobcov_lib). Usage: Crep(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 C-representation [P,Q] of the locally closed set, formed by a pair of radical ideals with P included in Q, representing the set V(P) \ V(Q) = V(N) \ V(M) 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-2;
    // C-representation of V(p) \ V(q)
    Crep(p,q);
    ==> [1]:
    ==>    _[1]=ab
    ==> [2]:
    ==>    _[1]=b-2
    ==>    _[2]=a

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