code: public ExpressionBuilder operator(Operator operator) {
          this.checkOperatorSymbol(operator);
          this.userOperators.put(operator.getSymbol(), operator);
          return this;
      }
docs: Add an {@link net.objecthunter.exp4j.operator.Operator} which should be available for use in the expression
  @param operator the custom {@link net.objecthunter.exp4j.operator.Operator} to add
  @return the ExpressionBuilder instance