def acos(x, err=defaultError):
"""
acos(x [,err]) returns arc cosine of x.
"""
if x > 1 or x < -1:
raise ValueError("%s is not in the range [-1, 1]." % str(x))
if x == 0:
return pi(err) / 2
if err <= defaultError:
rx = rational.Rational(x)
y = sqrt(1 - rx**2)
if rx > 0:
return asin(y, err)
else:
return pi(err) + asin(-y, err)
else:
return rational.Rational(math.acos(x))
