function searchLeft( x, p, lambda ) {
	while ( true ) {
		if ( x === 0 || cdf( x - 1.0, lambda ) < p ) {
			return x;
		}
		x -= 1;
	}
}
Performs a search to the left.

@private
@param {NonNegativeInteger} x - starting guess
@param {Probability} p - probability
@param {NonNegativeNumber} lambda - mean parameter
@returns {NonNegativeInteger} `p` quantile of the specified distribution