function logcdf( x, lambda ) {
	return addon( x, lambda );
}
Evaluates the natural logarithm of the cumulative distribution function (CDF) for an exponential distribution with rate parameter `lambda` at a value `x`.

@private
@param {number} x - input value
@param {PositiveNumber} lambda - rate parameter
@returns {number} evaluated logCDF

@example
var y = logcdf( 2.0, 0.1 );
// returns ~-1.708

@example
var y = logcdf( 1.0, 2.0 );
// returns ~-0.145

@example
var y = logcdf( -1.0, 4.0 );
// returns -Infinity

@example
var y = logcdf( NaN, 1.0 );
// returns NaN

@example
var y = logcdf( 0.0, NaN );
// returns NaN

@example
// Negative rate parameter:
var y = logcdf( 2.0, -1.0 );
// returns NaN