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The equivalent class of conformations. The union of the sites occupied by stars (*) is the fixed void υ. Here polymer a, b, c and d are different chains enclosing void υ, as indicated by the different sites occupied by the starting monomer x1 and the next monomer x2. However, the shapes of the polymers taken up by the union of the occupied sites for these chains are the same. As a consequence, these four polymers are equivalent.
