Hi Julien!

> I think you need to remove the apply, but keep one level of parenthesis, 
> like so:
> 
> ((assoc-ref …) args …)
> 
> Apply takes the last arguments, which must be a list and appends all its 
> elements to the invocation. Like (apply foo '(1 2 3 4)) is tge same as 
> (foo 1 2 3 4). Here you're probably confused because of the assoc-ref is 
> not a procodure, but it's fine: it returns a procedure, so guile can 
> execute it and pass arguments to it the normal way.

So I tried the following:

```
            (replace 'install
              (lambda _
                (for-each (lambda (solution)
                            (with-directory-excursion solution
                                 ((assoc-ref copy:%standard-phases 'install)
                                        #:install-plan
                                        '(("src" (string-append 
"include/" solution)
                                           #:include-regexp ("\\.h$"))))))
                          (list
                           "qtlockedfile"
                           "qtpropertybrowser"
                           "qtservice"
                           "qtsingleapplication"
                           "qtsoap"))
                #t))
```

I get "In procedure string-append: Wrong type (expecting string): #f", 
with and without replacing `(string-append "include/" solution)` with 
`"include"`.

Regards,
RG.