code: public static TableFactor logUnity(VariableNumMap vars) {
      TableFactorBuilder builder = new TableFactorBuilder(vars, SparseTensorBuilder.getFactory());
      return builder.buildSparseInLogSpace();
    }
docs: Gets a {@code TableFactor} over {@code vars} which assigns weight 1 to all
  assignments. The weights are represented in log space.
  
  @param vars
  @return