function spence( x ) {
	return addon( x );
}
Evaluates Spence's function, which is also known as the dilogarithm.

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

@example
var v = spence( 3.0 );
// returns ~-1.437

@example
var v = spence( 0.0 );
// returns ~1.645

@example
var v = spence( -9.0 );
// returns NaN

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