code: public DynamicVariableSet addFixedVariables(VariableNumMap variables) {
      Preconditions.checkArgument(!fixedVariables.containsAny(variables));
  
      return new DynamicVariableSet(fixedVariables.union(variables), plateNames, plates,
          maximumReplications);
    }
docs: Returns a copy of {@code this} with an additional fixed
  variables. The added variables use the names and numbers provided
  in {@code variables}.
  
  @param variables
  @return