function logcdf( x ) {
		if ( x < 0.0 ) {
			return NINF;
		}
		return log1p( -exp( -lambda * x ) );
	}
Evaluates the natural logarithm of the cumulative distribution function (CDF) for an exponential distribution.

@private
@param {number} x - input value
@returns {number} evaluated logCDF

@example
var y = logcdf( 2.0 );
// returns <number>