code: public void remove(int id) {
      String symbol = invert().keyForId(id);
      idToSymbol.remove(id);
      symbolToId.remove(symbol);
    }
docs: Remove the mapping for the given id. Note that the newly assigned 'next' ids are monotonically
  increasing, so removing one id does not free it up to be assigned in future symbol table adds; there
  will just be holes in the symbol mappings
  @see #trimIds() for a way to compact the assigned ids
  @param id