Tries to return the one and only element of the sequence.
// 1
Enumerable.create(1)
.singleOrDefault();
// "TM"
Enumerable.create()
.singleOrDefault('TM');
// "YS"
Enumerable.create(11)
.singleOrDefault('x => typeof x === "string"',
'YS');
// "JS"
Enumerable.create(111, 222)
.singleOrDefault('x => typeof x === "string"',
'JS');
// 2222
Enumerable.create(1111, 2222)
.singleOrDefault('x => x > 1111',
'MK');
// THROWS EXCEPTION
Enumerable.create(11111, 22222, 33333)
.singleOrDefault('x => x > 11111',
'PZ');