code: private String getBaseSymbol() {
      String ncNotation = getNotation();
      if (!ncNotation.contains("(")) {
        return null;
      }
      int start = ncNotation.indexOf("(");
      int end = ncNotation.indexOf(")");
  
      String baseSymbol = ncNotation.substring(start + 1, end);
      baseSymbol = baseSymbol.replaceAll("\\[|\\]", "");
      return baseSymbol;
    }
docs: find the base symbol, if it is a modified monomer, the "[" and "]" will be
  removed
  
  @return base monomer ID