@Override
    public String toString() {
        String operationString = "<unknown command>";
        
        switch (operation) {
                case EQUAL:
                    operationString = "==";
                    break;
                case GREATER:
                    operationString = ">";
                    break;
                case LESS:
                    operationString = "<";
                    break;
                case GREATER_OR_EQUAL:
                    operationString = ">=";
                    break;
                case LESS_OR_EQUAL:
                    operationString = "<=";
                    break;
                case NOT_EQUAL:
                    operationString = "!=";
                    break;
            }
        
        return left.toString() + " " + operationString + " " + right.toString();
    }