I want you to act as a unit test generator. 
Your role is to generate unit test code used TypeScript language and NAME_1 test framework. 
I want you to only generate the code output in a unique code block and nothing else, don't write explanations.
The _StringEqualityComparer source code:
``` typescript
namespace dv {
    /**
     * @hidden
     * Represents the equality comparer of string value.
     */
    export class _StringEqualityComparer implements IEqualityComparer<string | null> {
        /**
         * @hidden
         * Gets the equality comparer of string value.
         */
        public static readonly _stringEqualityComparer = new _StringEqualityComparer();

        /**
         * @hidden
         * Gets the array equality comparer of string value.
         */
        public static readonly _stringArrayEqualityComparer = new _ArrayEqualityComparer<string>(_StringEqualityComparer._stringEqualityComparer);

        //#region IEqualityComparer implementation

        /**
         * @hidden
         * Check the two object whether equal.
         * @param a The a object
         * @param b the b object
         * @returns If the two key are equal true otherwise false.
         */
        public _equalsWith(a: string | null, b: string | null): boolean {
            if ((a == null) && (b == null)) {
                return true;
            } else if ((a == null) || (b == null)) {
                return false;
            } else {
                return a === b;
            }
        }

        //#endregion
    }
}
```
The NAME_1 code sample, please complete the unit test function body and return them.
``` typescript
describe('_StringEqualityComparer', () => {
    let instance: dv._StringEqualityComparer;

    beforeEach(() => {
        instance = new dv._StringEqualityComparer();
    });


    it('method _stringEqualityComparer()', () => {
    });

    it('method _stringArrayEqualityComparer()', () => {
    });

    it('method _equalsWith()', () => {
    });

});
``` In your entire response, the letter y should appear less than 16.
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