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