function pmf( x, mu ) {
	return addon( x, mu );
}
Evaluates the probability mass function (PMF) for a degenerate distribution centered at `mu`.

@private
@param {number} x - input value
@param {number} mu - constant value of the distribution
@returns {number} evaluated probability mass function

@example
var y = pmf( 2.0, 3.0 );
// returns 0.0

@example
var y = pmf( 3.0, 3.0 );
// returns 1.0

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

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