resource Stack
  type result = enum(OK, OVERFLOW, UNDERFLOW)
  op push(int item) returns result  r
  op pop(res int item) returns result  r
body Stack(int size)
  int store[1:size]
  int top = 0
  proc push(item) returns r  {
    if (top<size) { store[++top] = item; r = OK
    } else if (top==size ) { r = OVERFLOW
    }
  }
  proc pop(item) returns r  {
    if (top>0) { item = store[top--]; r = OK
    } else if (top==0 ) { r = UNDERFLOW
    }
  }
end Stack