function pmf( x ) {
		var lnl;
		if ( isnan( x ) ) {
			return NaN;
		}
		if ( isNonNegativeInteger( x ) ) {
			if ( x > n ) {
				return 0.0;
			}
			lnl = binomcoefln( n, x );
			lnl += (x * ln( p )) + ((n - x) * log1p( -p ));
			return exp( lnl );
		}
		return 0.0;
	}
Evaluates the probability mass function (PMF) for a binomial distribution.

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

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