GAP 4.8.9 installation with standard packages -- copy to your CoCalc project to get it

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/****************************************************************************
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**
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*A  down_class.c                ANUPQ source                   Eamonn O'Brien
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**
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*Y  Copyright 1995-2001,  Lehrstuhl D fuer Mathematik,  RWTH Aachen,  Germany
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*Y  Copyright 1995-2001,  School of Mathematical Sciences, ANU,     Australia
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**
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*/
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#include "pq_defs.h"
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#include "pcp_vars.h"
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/* remove any class pcp->cc + 1 part on the value represented by y[ptr] */
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void down_class(int ptr, struct pcp_vars *pcp)
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{
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   register int *y = y_address;
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   register int p1;
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   register int p2;
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   register int count;
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   register int count1;
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   register int lastg = pcp->lastg;
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#include "access.h"
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   p1 = ptr;
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   if (y[p1] >= 0) {
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      if (y[p1] > lastg)
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         y[p1] = 0;
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      return;
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   }
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   p2 = -y[p1];
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   count = y[p2 + 1];
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   count1 = count;
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   while (FIELD2(y[p2 + count1 + 1]) > lastg) {
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      if (--count1 <= 0) {
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         y[p2] = y[p1] = 0;
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         return;
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      }
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   }
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   if (count1 >= count)
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      return;
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   if (count == count1 + 1)
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      /* only 1 generator of class pcp->cc + 1 found; mark
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         it with a -1 header block */
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      y[p2 + count1 + 2] = -1;
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   else {
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      y[p2 + count1 + 2] = 0;
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      y[p2 + count1 + 3] = count - count1 - 2;
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   }
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   y[p2 + 1] = count1;
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}
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