at least until the object is not intended to do copying the structure of
seq[seq[int]] will copy its children seq

var x = @[@[1, 2], @[1]]
var y = x
x.add @[1]
echo x, y
x[0][0] = 42
echo x, y

@[@[1, 2], @[1], @[1]]@[@[1, 2], @[1]]
@[@[42, 2], @[1], @[1]]@[@[1, 2], @[1]]

type
  A = object
    x: B
  B = ref object
    x: int
var x = @[A(x: B(x: 10))]
var y = x
echo repr(x), repr(y)
x[0].x.x = 32
echo repr(x), repr(y)

@[A(x: B(x: 10))]@[A(x: B(x: 10))]
@[A(x: B(x: 32))]@[A(x: B(x: 32))]