function log1pexp( x ) {
	if ( isnan( x ) ) {
		return NaN;
	}
	if ( x <= -37.0 ) {
		return exp( x );
	}
	if ( x <= 18.0 ) {
		return log1p( exp( x ) );
	}
	if ( x <= 33.3 ) {
		return x + exp( -x );
	}
	// Case: x > 33.3
	return x;
}
Computes the natural logarithm of \\( 1 + \exp(x) \\).

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

@example
var v = log1pexp( 1.1 );
// returns ~1.387

@example
var v = log1pexp( 100.0 );
// returns 100.0

@example
var v = log1pexp( NaN );
// returns NaN