acosh returns the hyperbolic arc cosine of the argument. The return
value is expressed in radians. If the argument is a matrix, the operation
is performed element-by-element.
This is not an built-in function. This function
is normally loaded on start-up from the acosh.r file in the
standard rlib directory. Use of the -r option, incorrectly
setting the RLAB_LIB_DIR environmental variable, or modifying
acosh.r may make this function unavailable.