code: public static FrozenSymbolTable freeze(SymbolTable symbolTable) {
      if (symbolTable instanceof FrozenSymbolTable) {
        return (FrozenSymbolTable) symbolTable;
      }
      return new FrozenSymbolTable(symbolTable);
    }
docs: Freeze the given symbol table by decorating it (or just returning if its already frozen)
  @param symbolTable
  @return