====== Description ======
The acos node returns the arc cosine of the value of its child-node.

====== Constraints ======
  * Minimum number of child-nodes: 1
  * Maximum number of child-nodes: 1
  * Valid input range: -1. to +1.
  * Output range: 0 to <m>pi</m> (radians) 

====== Calculus ======
The acos node's value is the arc cosine, in radians, of the value of its child-node. If the input value is out of range then the acos node returns an ERROR value.

For valid value<sub>1</sub>:

value = cos<sup>-1</sup>(value<sub>1</sub>)

For invalid value<sub>1</sub>:

value = ERROR