function logcdf( x ) {
		if ( x <= 0.0 ) {
			return NINF;
		}
		if ( x === PINF ) {
			return 0.0;
		}
		return ln( gammainc( x * beta, alpha ) );
	}
Evaluates the logarithm of the cumulative distribution function (CDF) for a gamma distribution.

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

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