code: public Object unbindName(String name, IndexedList<String> symbolTable) {
      if (!symbolTable.contains(name)) {
        return null;
      }
      int index = symbolTable.getIndex(name);
      return bindings.remove(index);
    }
docs: Removes a binding from this environment. Returns the value
  bound to that name if the name was bound, otherwise returns
  {@code null}.
  
  @param name
  @param symbolTable
  @return