code: public DynamicVariableSet addFixedVariable(String name, Variable variable) {
      int variableIndex = fixedVariableMaxInd + 1;
      return new DynamicVariableSet(fixedVariables.addMapping(variableIndex, name, variable),
          plateNames, plates, maximumReplications);
    }
docs: Returns a copy of {@code this} with an additional fixed variable.
  The created variable is named {@code name} and has type
  {@code variable}.
  
  @param name
  @param variable
  @return