That is not exactly true. The first time you used the word template was quite
some way in when you assumed that you’d need to set the markups differently
for any possible configuration (which is where I answered you’d be
underestimating Lilypond as you do NOT need to do that. My example showed a
way you can have ONE header/footer markup producing different results on
global flags (these could also be put inside the header block or a paper
block), basically showing you how to create a simple interface like you wanted
(although by that point it was not clear to me that you wanted that)).
... but, if you pick up again that message, you will find that the interface you created did not meet my specs. Here are my words:
"I should have the flexibility to switch on the fly from one choice to another, which is expected in a header + body + footer template, therefore this template should not be polluted by mixing body with footer.". In fact you redefined the template without wrapping it. Then, it should not be considered a template with the flexibilty to switch from one choice to another, but rather a customization of a template that doesn't offer that easy switch. Even if you do the easy switch as a result, this can't be used as template because you did not wrap it. And this has disadvantages, as I tried to explain, adding that I could wrap it in my own project, so to meet my specs, but this procedure would have disadvantages too.
The technical problem here is that there is no clear border between body,
header and footer (unless we are taking full control of the page, for which I
gave you a way to do this without footer (as I said before, I did not realize
we were talking about cover pages before that, even though it says so in the
subject. I was expecting a first page with music on it)).
This is not true, and this is what I added in my explanation later. There is such clear border: it is given by the template defaults. These are the rules for body, header and footer. When you decide to modify these rules, then you are creating a grey area, then it is needed to wrap your customization, so to create new defaults (and, consequently, a new template). When you violate this rule by modifying two instances of two implementations (footer and header) of an interface, instead of creating new instances, then you are doing a hack which makes the code even dirtier, then a wrap is doubly needed. In the example we are discussing, the motto is logically part of the body. Then, given that the LP template doesn't meet this specs, a new template should be created by wrapping a customization of the original one. Then you have clean code and you can have new clear borders. As you can see, this is a tedious procedure, and this is why I prefer to use alternative tools.
But let’s be specific about the problem: So your aim here is to create
templates? Or do just want to create a cover page with something at the
my aim (which is not what I asked at the beginning of the thread, though) is not to create templates (I don't have time for now), but to have a way to have them for the next future. And I got this result with the alternative tool. And, of course, with the help of your (and Aaron's) examples, which made me understand how the things works in LP. Therefore you are wrong when you feel unmotivated for the things you read. You should consider the opposite...
Please note that there's nothing strange in using an alternative tool for this. And I would leave the LP code totally untouched for these tasks. IMHO LP doesn't need to have such sophisticated tools for cover pages, as Jean wished. It would be totally beyond its scope. Therefore I find ridiculous (and pathetic at the same time) when my words appear as a blame.