function logcdf( x, alpha, beta ) {
	return ln( cdf( x, alpha, beta ) );
}
Evaluates the logarithm of the cumulative distribution function (CDF) for a gamma distribution with shape parameter `alpha` and rate parameter `beta` at a value `x`.

@param {number} x - input value
@param {NonNegativeNumber} alpha - shape parameter
@param {PositiveNumber} beta - rate parameter
@returns {number} evaluated logCDF

@example
var y = logcdf( 2.0, 0.5, 1.0 );
// returns ~-0.047

@example
var y = logcdf( 0.1, 1.0, 1.0 );
// returns ~-2.352

@example
var y = logcdf( -1.0, 4.0, 2.0 );
// returns -Infinity

@example
var y = logcdf( NaN, 0.6, 1.0 );
// returns NaN

@example
var y = logcdf( 0.0, NaN, 1.0 );
// returns NaN

@example
var y = logcdf( 0.0, 1.0, NaN );
// returns NaN

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

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