Removing whitespace duplicates

["Nordlöw" via Digitalmars-d-learn]

How can I extend

string line = "car    wash";

line.strip.splitter!isWhite.joiner("_").to!string

so that

car   wash

becomes

car_wash

and not

car___wash

?

Used at https://github.com/nordlow/justd/blob/master/knet.d#L1142