function log1pexp( x ) {
	if ( x <= 18.0 ) {
		return log1p( exp(x) );
	}
	if ( x > 33.3 ) {
		return x;
	}
	// Case: 18.0 < x <= 33.3
	return x + exp( -x );
}
Compute ln( 1 + exp(x) ) without overflow.

@private
@param {number} x - input value
@returns {number} function value