Removing whitespace duplicates

["Nordlöw" via Digitalmars-d-learn]

On Tuesday, 21 October 2014 at 08:04:13 UTC, bearophile wrote:
> If your text is ASCII, then there is std.ascii.whitespace that 
> can be given as argument to std.string.tr.
>
> Bye,
> bearophile

http://dlang.org/phobos/std_ascii.html#.whitespace

is of string but

std.string.tr needs [string] as argument.

How does that work?