function quantile( p, mu ) {
	return addon( p, mu );
}
Evaluates the quantile function for a degenerate distribution centered at `mu`.

@private
@param {Probability} p - input value
@param {number} mu - constant value of the distribution
@returns {number} evaluated quantile function

@example
var y = quantile( 0.5, 2.0 );
// returns 2.0

@example
var y = quantile( 0.9, 4.0 );
// returns 4.0

@example
var y = quantile( 1.1, 0.0 );
// returns NaN

@example
var y = quantile( -0.2, 0.0 );
// returns NaN

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

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