Thank you!

To convert the value of w to string, your mixin
uses write.

Should I use something with format: 

 auto newstring = part ~ format("%s",w) ~ other_part;

or 

 auto newstring =format("....part %s ...other..part",w);

to be sure to have an conversion to string available,
because I do not want to write?

Personally, I'm pretty happy with the printf inspired syntax that is
supported by format/writefln/formattedWrite. It also supports
positional arguments for things like i18n strings.

p.s. When I use tab in the Browser (Firefox latest@Ubuntu) while editing this form, the left edited part grows many folds.