func BackIn(t, b, c, d float64) float64 {
	if t > d {
		return c
	}
	s := math.SqrtPi
	t /= d
	return c*t*t*((s+1)*t-s) + b
}
////////////////////////////////////////////////////////////////////////////////// //
BackIn Accelerating from zero velocity
https://easings.net/#easeInBack
package easing
import (
	"math"
)