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

@private
@param {number} x - input value
@param {Probability} p - success probability
@returns {NonPositiveNumber} logarithm of PMF

@example
var y = logpmf( 4.0, 0.3 );
// returns ~-2.631

@example
var y = logpmf( 2.0, 0.7 );
// returns ~-2.765

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

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

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

@example
// Invalid success probability:
var y = logpmf( 2.0, 1.5 );
// returns NaN