How to define a slice in a circuit struct like the following:

[George551556]

How to define a slice in a circuit struct like the following and perform operations in the definition? Why do I get a panic error with the message 'unrecognized R1CS curve type' when I use the following code at groth16.Setup(ccs)?

package testing

import (
	"fmt"

	"github.com/consensys/gnark-crypto/ecc"
	"github.com/consensys/gnark/backend/groth16"
	"github.com/consensys/gnark/frontend"
	"github.com/consensys/gnark/frontend/cs/r1cs"
)

type Circuit struct {
	Arr    []frontend.Variable
	Output frontend.Variable `gnark:",public"`
}

func (circuit *Circuit) Define(api frontend.API) error {
	api.AssertIsEqual(api.Add(circuit.Mat[0], circuit.Mat[1]), circuit.Output)
	return nil
}

func main() {
	var circuit Circuit
	ccs, _ := frontend.Compile(ecc.BN254.ScalarField(), r1cs.NewBuilder, &circuit)
	pk, vk, _ := groth16.Setup(ccs)

	assignment := Circuit{
		Arr:    []frontend.Variable{1, 1},
		Output: 2,
	}
	witness, _ := frontend.NewWitness(&assignment, ecc.BN254.ScalarField())

	proof, _ := groth16.Prove(ccs, pk, witness)
	publicWitness, _ := witness.Public()

	err := groth16.Verify(proof, vk, publicWitness)
	if err == nil {
		fmt.Println("success")
	}
}