Computes the arccosine (in radians) of the specified argument.

Format: Acos(exprn)

Arguments:

- exprn (float): Expression to compute the arccosine of.

Return type: float

Error conditions: Returns 'Bottom' if exprn evaluates to 'Bottom', or if exprn is not in the domain of arccosine (which is [-1.0,1.0]).

Example:

`value = acos(1)
; Sets value to the arccosine of 1, which is approximately 0.0 (possibly with some rounding error.)`