function log1pmx( x ) {
	return addon( x );
}
Evaluates \\( \operatorname{log1pmx}(x) = \ln(1+x) - x \\).

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

@example
var v = log1pmx( 1.1 );
// returns ~-0.358

@example
var v = log1pmx( 0.99 );
// returns ~-0.302

@example
var v = log1pmx( -0.99 );
// returns ~-3.615

@example
var v = log1pmx( -1.1 );
// returns NaN

@example
var v = log1pmx( NaN );
// returns NaN