
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
The expressions  $ a \plus{} b \plus{} c, ab \plus{} ac \plus{} bc,$  and  $ abc$  are called the elementary symmetric expressions on the three letters  $ a, b, c;$  symmetric because if we interchange any two letters, say  $ a$  and  $ c,$  the expressions remain algebraically the same. The common degree of its terms is called the order of the expression. Let  $ S_k(n)$  denote the elementary expression on  $ k$  different letters of order  $ n;$  for example  $ S_4(3) \equal{} abc \plus{} abd \plus{} acd \plus{} bcd.$  There are four terms in  $ S_4(3).$  How many terms are there in  $ S_{9891}(1989)?$  (Assume that we have  $ 9891$  different letters.)

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