function searchLeft( x, r, n, p ) {
	while ( x !== 0 && cdf( x-1, n, p) >= r ) {
		x -=1;
	}
	return x;
}
Performs a search to the left.

@private
@param {NonNegativeInteger} x - starting guess
@param {Probability} r - probability
@param {NonNegativeInteger} n - number of trials
@param {Probability} p - success probability
@returns {NonNegativeInteger} `r` quantile of the specified distribution