Debussy have wished a polyrhythmic or free feeling: the clouds are running in triplets in 6/4 against the englishhorns tuplets
I have tried alt-instert of 4/4 (C) in the English-horn. Wrong.
But how do I notate like Debussy ?
You need to use hidden tuplets for this.
O.K. But what about the 4/4 (C) symbol?
Create a time signature of C with a pickup bar 6 beats long. Then change the time signature back to 6/4 in the next bar and hide it.
Select the bar, press M, then type C,6 Alt-Enter in the popover so the time signature only applies to one staff.
Thanks to both of you. It works (- of course 
)
Impressing what can be done in Dorico.
It worked fine. Thanks.