function betaln( a, b ) {
	return addon( a, b );
}
Evaluates the natural logarithm of the beta function.

@private
@param {NonNegativeNumber} a - first input value
@param {NonNegativeNumber} b - second input value
@returns {number} natural logarithm of beta function

@example
var v = betaln( 0.0, 0.0 );
// returns Infinity

@example
var v = betaln( 1.0, 1.0 );
// returns 0.0

@example
var v = betaln( -1.0, 2.0 );
// returns NaN

@example
var v = betaln( 5.0, 0.2 );
// returns ~1.218

@example
var v = betaln( 4.0, 1.0 );
// returns ~-1.386

@example
var v = betaln( NaN, 2.0 );
// returns NaN