how to use new markup scheme type ???

[Alexandre Beneteau]

Hello,

Since lily 1.7.13, I used a usefull macro to instruct brass instruments 
to put mute on :

sourdineOn = \notes { \once \property Voice.MultiMeasureRestNumber 
\override #'text = #'(roman "Mettez la sourdine") }

Then, in the score, just before multi measure rest :
\sourdineOn R1*12

But, now lily 1.7.15 complain about the type of argument :

Type check for `text' failed; value `(roman "Mettez la sourdine")' must 
be of type `markup

I've tried to dig in the documentation and even the scm files.... but 
can figure out how to well format a markup type (scheme is quite obscur 
at that time for me...)

I suppose this is a trivial for several of you, but I would be very 
happy if someone could quick show me how to proceed...

Thanks in advance,

        Alex.