/sys$common/syshlp/HELPLIB.HLB  —  CRTL  acosh  Description
    The acosh functions return the hyperbolic arc cosine of x for x
    in the domain [1, +Infinity], where acosh(x) = ln(x + sqrt(x**2 -
    1)).

    The acosh function is the inverse function of cosh where
    acosh(cosh(x)) = |x|.

    x < 1 is an invalid argument.
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