Groups the elements of an observable sequence according to a specified key selector function and comparer and selects the resulting elements by using a specified function.
Function): A function to extract the key for each element.
Function): A function to map each source element to an element in an observable group.
Any): Used to compare objects. If not specified, the default comparer is used.
Observable): A sequence of observable groups, each of which corresponds to a unique key value, containing all elements that share that same key value.