Return the arc cosine of {expr} measured in radians, as a |Float| in the range of [0, pi]. {expr} must evaluate to a |Float| or a |Number| in the range [-1, 1]. Otherwise acos() returns "nan". Examples: :echo acos(0) 1.570796 :echo acos(-0.5) 2.094395 Can also be used as a |method|: Compute()->acos() {only available when compiled with the |+float| feature}