<commit_before>### <em>function</em> <strong>`acos`</strong>

Syntax:

<strong>`acos`</strong> <em>parameters</em> => <em>return-type</em>

Documentation of parameters and return-results.

Examples (not from CLHS...):

```lisp
CL-USER> (example-code 'a 'b 'c)

'return-result
```
<commit_msg>Add documentation for ACOS, inluding ASCIIMathML formal definitions<commit_after>### <em>function</em> <strong>`acos`</strong>

**Syntax:** <strong>`acos`</strong> <em>number</em> => <em>number</em>

**Description:**

`acos` returns the arc cosine of the argument *number*. The result is in radians. The argument may be complex.

The arc cosine function may be defined mathematically for an argument *z* as follows:

\`-i log(z + root i (1-z^2))\`

Note that the result of `acos` may be complex even if the argument is not complex; this occurs when the absolute value of the argument is greater than 1.

Alternate suggestions for the defining formula are:

\`(2 log(sqrt(1+z/2)+root i (1-z/2)))/i\`

Or the much simpler, \`(pi/2)-arcsin z\`

Both equations are mathematically equivalent to the formula shown above.

**Examples:**

```lisp
CL-USER> (example-code 'a 'b 'c)

'return-result
```

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{'dir': 'git-commits-cleaned', 'id': '2924202', 'n_tokens_mistral': 416, 'n_tokens_neox': 395, 'n_words': 140}