===== Arc Cosine ======

==== acos( a ) ====

Return the arc cosine of a.

=== Description ===

<code>acos( a )</code>

Return the arc cosine of a in radians.

=== Parameters ===

| ''a'' | Value [-1 to +1] |

=== Return value ===

arc cosine of a

=== Example usage ===

<code>acos( 1 )</code>

Returns 0.