split with delimiter

[bioinfornatics]

reading
http://www.d-programming-language.org/phobos/std_array.html#split

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S[] split(S)(S s); // merge space together

and

Un­qual!(S1)[] split(S1, S2)(S1 s, S2 delim); // do not merge delim
together ?

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why the second split function do not merge delim together?
how merge delim together?