Hi Ricardo!

> As Julien wrote this is expected as you have an expression that starts
> with a string:
> 
>     ("hello" whatever)
> 
> This will try to apply the string "hello" to the argument “whatever”.  I
> don’t know how to apply a string to anything, and neither does Scheme.
> 
> I would go with quasiquotation:
> 
> --8<---------------cut here---------------start------------->8---
> (with-directory-excursion solution
>    ((assoc-ref copy:%standard-phases 'install)
>     #:install-plan
>     `(("src" ,(string-append "include/" solution)
>        #:include-regexp ("\\.h$")))))
> --8<---------------cut here---------------end--------------->8---
> 
> The expression following #:install-plan is quoted (so it’s inert data),
> and we temporarily unquote to evaluate the “string-append” expression;
> then we go straight back to “data mode”, so we don’t have to explicitly
> quote “("\\.h$")”.

It worked. Thanks so much!

Regards,
RG.