function lower( x, n, alpha ) {
	return ( x === 0 ) ? 0 : betaQuantile( alpha, x, n - x + 1 );
}
Calculates the lower endpoint of a confidence interval.

@private
@param {NonNegativeInteger} x - number of successes
@param {NonNegativeInteger} n - total number of observations
@param {number} alpha - significance level
@returns {number} lower endpoint