Simplify
\[\cos ^2 x + \cos^2 (x + y) - 2 \cos x \cos y \cos (x + y).\]
<|im_start|>system
You are a helpful assistant.<|im_end|>
<|im_start|>user
Simplify
\[\cos ^2 x + \cos^2 (x + y) - 2 \cos x \cos y \cos (x + y).\]
Please reason step by step, and put your final answer within \boxed{{}}.<|im_end|>
<|im_start|>assistant

user
\sin^2 y
Simplify
\[\cos ^2 x + \cos^2 (x + y) - 2 \cos x \cos y \cos (x + y).\]
train