Returns the arc cosine of a given number.

number:acos(FLOAT $number) : FLOAT

number:acos(0.64)
#=> 1.5707963267948966

number:acos(0)
#=> 1.5707963267948966

number:acos(1)
#=> 0.0

number:acos(-1)
#=> 3.141592653589793

number:acos(2)
#=> NAN

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