  
  [1X12 [33X[0;0YStacks[133X[101X
  
  [33X[0;0YA  stack  is  a  deque where items can be [10XPush[110Xed onto the stack, and the top
  item can be [10XPop[110Xped off the stack.[133X
  
  [33X[0;0YStacks are wrapped GAP plain lists.[133X
  
  
  [1X12.1 [33X[0;0YAPI[133X[101X
  
  [1X12.1-1 Stack[101X
  
  [33X[1;0Y[29X[2XStack[102X(  ) [32X function[133X
  [6XReturns:[106X  [33X[0;10Ystack[133X
  
  [33X[0;0YConstructor for stacks[133X
  
  [1X12.1-2 IsStack[101X
  
  [33X[1;0Y[29X[2XIsStack[102X( [3Xarg[103X ) [32X filter[133X
  [6XReturns:[106X  [33X[0;10Y[9Xtrue[109X or [9Xfalse[109X[133X
  
  [33X[0;0YCategory of heaps[133X
  
  [1X12.1-3 Push[101X
  
  [33X[1;0Y[29X[2XPush[102X( [3Xstack[103X, [3Xobject[103X ) [32X operation[133X
  
  [33X[0;0YPuts [3Xobject[103X onto [3Xstack[103X.[133X
  
  [1X12.1-4 Peek[101X
  
  [33X[1;0Y[29X[2XPeek[102X( [3Xstack[103X ) [32X operation[133X
  [6XReturns:[106X  [33X[0;10Yobject or [9Xfail[109X[133X
  
  [33X[0;0YReturn the object at the top of [3Xstack[103X. If [3Xstack[103X is empty, returns [9Xfail[109X[133X
  
  [1X12.1-5 Pop[101X
  
  [33X[1;0Y[29X[2XPop[102X( [3Xstack[103X ) [32X operation[133X
  [6XReturns:[106X  [33X[0;10Yobject or [9Xfail[109X[133X
  
  [33X[0;0YRemove  the  top  item  from  [3Xstack[103X  and  return it. If [3Xstack[103X is empty, this
  function returns fail.[133X
  
  [1X12.1-6 Size[101X
  
  [33X[1;0Y[29X[2XSize[102X( [3Xarg[103X ) [32X attribute[133X
  
  [33X[0;0YNumber of elements on [3Xstack[103X[133X
  
