Question: simple question about subs

Hi there

I'm taking my first steps with maple and my question is not exactly briliiant

Putting in a simple way, I'm trying to solve sth that looks like this

> li := a*[cos(7*x+8*x*y+9*y)*cos(-7*x-8*x*y-9*y)];
             a [cos(7 x + 8 x y + 9 y) cos(-7 x - 8 x y - 9 y)]

and i want to replace

with

getting (if i do it by hand)

> algsubs(7*x+8*x*y+9*y = 20*y, li);
                    a [cos(20 y) cos(-7 x - 8 x y - 9 y)]

my question, is there an elegant way to do it? sth that would look like this?

 

 

thank you for any tips

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