Computes the arccosine (in radians) of the specified argument.
Expression to compute the arccosine of (float).
Returns bottom if exprn evaluates to bottom, or if exprn is not in the domain of arccosine (which is [-1.0,1.0]).
value = acos(1) (possibly with some rounding error.)
Sets value to the arccosine of 1, which is approximately 0.0