function isProbabilityf( x ) {
	return Boolean( addon( x ) );
}
Tests if a single-precision floating-point number is a probability.

@private
@param {number} x - value to test
@returns {boolean} boolean indicating whether the value is a probability

@example
var bool = isProbabilityf( 0.5 );
// returns true

@example
var bool = isProbabilityf( 3.14 );
// returns false

@example
var bool = isProbabilityf( NaN );
// returns false