code: function pmf( x, p ) {
  	var q;
  	if ( x !== x) {
  		return NaN;
  	}
  	if ( isNonNegativeInteger( x ) ) {
  		q = 1 - p;
  		return p * pow( q, x );
  	}
  	return 0;
  }
docs: PMF // 
  FUNCTION: pmf( x, p )
  Evaluates the probability mass function (PMF) for a Geometric distribution with success probability `p` at a value `x`.
  
  @param {Number} x - input value
  @param {Number} p - success probability
  @returns {Number} evaluated PMF