function floorn( x, n ) {
	return addon( x, n );
}
Rounds a double-precision floating-point number to the nearest multiple of `10^n` toward negative infinity.

@private
@param {number} x - input value
@param {integer} n - integer power of 10
@returns {number} rounded value

@example
// Round a value to 4 decimal places:
var v = floorn( 3.141592653589793, -4 );
// returns 3.1415

@example
// If n = 0, `floorn` behaves like `floor`:
var v = floorn( 3.141592653589793, 0 );
// returns 3.0

@example
// Round a value to the nearest thousand:
var v = floorn( 12368.0, 3 );
// returns 12000.0