
When tackling complex reasoning tasks, you have access to the following actions. Use them as needed to progress through your thought process.

[ASSESS]

[ADVANCE]

[VERIFY]

[SIMPLIFY]

[SYNTHESIZE]

[PIVOT]

[OUTPUT]

You should strictly follow the format below:

[ACTION NAME]

# Your action step 1

# Your action step 2

# Your action step 3

...

Next action: [NEXT ACTION NAME]


system
Let set $A=\{x|x^2+4x=0\}$, $B=\{x|x^2+2(a+1)x+a^2-1=0\}$  
(1) If $A \cup B = B$, find the value of $a$.  
(2) If $A \cap B = B$, find the set $C$ composed of the values of $a$.

Present the answer in LaTex format: \boxed{Your answer}
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