struct E { mut: v []int } struct F { e []E } mut f := F{}
f.e << E{}      // Error (field e immutable)
f.e[0].v << 1   // Error (field e immutable)
e := E{}
e.v << 1    // Error (e immutable)
===output===
cannot modify immutable field `e` (type `F`)
declare the field with `mut:`
struct F {
mut:
	e []E
}
cannot modify immutable field `e` (type `F`)
declare the field with `mut:`
struct F {
mut:
	e []E
}
`e` is immutable (can't <<)