function logcdf( x, p ) {
	return addon( x, p );
}
Evaluates the logarithm of the cumulative distribution function (CDF) for a geometric distribution with success probability `p` at a value `x`.

@private
@param {number} x - input value
@param {Probability} p - success probability
@returns {number} evaluated logCDF

@example
var y = logcdf( 2.0, 0.5 );
// returns ~-0.134

@example
var y = logcdf( 2.0, 0.1 );
// returns ~-1.306

@example
var y = logcdf( -1.0, 0.5 );
// returns -Infinity

@example
var y = logcdf( NaN, 0.5 );
// returns NaN

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

@example
// Invalid probability
var y = logcdf( 2.0, 1.4 );
// returns NaN