Dynamic Template-Expansion of (Token) Strings

[Per Nordlöw]

I'm looking for elegant ways of expressing expansion of 
parameterized strings written to a file at run-time. My primary 
use case is run-time generation of D code. In the lazy case, 
something like

     import std.file : write;
     import std.algorithm : substitute;
     const s = q{int $X = $Y;}; // templated source
     foreach (part; s.substitute("$X", "x", "$Y", "42"))
         write(somePath, part);

is my current best bet.