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A function that can be used in Formulas.
arccosh (x)
x.| arccosh(x) = ln (x + √(x2−1)) | 
   assert arccosh (1.0) = 0
   assert arccosh (0.9) = undefined
   assert arccosh (0.0) = undefined
   assert arccosh (-10.0) = undefined
   assert arccosh (undefined) = undefined
© Paul Boersma 2023