#### D.15.1.19 arrRestrict

Procedure from library `arr.lib` (see arr_lib).

Return:
  arr: the restricted hyperplane Arrangement (A^X)

Note:
  A has to be non-empty.

Remarks:
  We restrict A to the flat X, defined by the equations in A[v]. The restriction will only be performed, if the ideal defining the flat X is monomial (i.e. X is an intersection of coordinate planes). If the optional argument CC is given, the arrangement is transformed in such a way that X has the above form.

Example:

    LIB "arr.lib";
    ring S = 0,(x,y,z),dp;
    arr A  = arrTypeB(3);
    A;
    ==> _[1]=x-y
    ==> _[2]=x+y
    ==> _[3]=x-z
    ==> _[4]=x+z
    ==> _[5]=x
    ==> _[6]=y-z
    ==> _[7]=y+z
    ==> _[8]=y
    ==> _[9]=z
    ==>
    arrRestrict(A,9);
    ==> _[1]=x-y
    ==> _[2]=x+y
    ==> _[3]=x
    ==> _[4]=y
    ==>
    arrRestrict(A,4,"CC");
    ==> _[1]=1/2y-z
    ==> _[2]=1/2y+z
    ==> _[3]=y
    ==> _[4]=z
    ==>
    intvec v=5,8;
    arrRestrict(A,v);
    ==> _[1]=-z
    ==>

See also: arrCone; arrDecone; arrEssentialize; arrIsEssential; arrRestrict.

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