tanh returns the hyperbolic tangent of the argument, which is
taken to be 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 tanh.r file in the
standard rlib directory. Use of the -r option, incorrectly
setting the RLAB_LIB_DIR environmental variable, or modifying
tanh.r may make this function unavailable.