EKURTOSIS // 
FUNCTION ekurtosis( p )
Computes the distribution ekurtosis for a geometric distribution with parameter p.

@param {Number} p - success probability
@returns {Number} distribution ekurtosis
function ekurtosis( p ) {
	if ( !( isNumber(p) && 0 <= p && p <= 1) ) {
		return NaN;
	}
	return 6 + square( p ) / ( 1 - p );
}