How to Override Schema Fields in Effect Typescript

export class A extends Schema.Class<A>("A")({
  a: Schema.Struct({
    x: Schema.String
  }),
  b: Schema.String
}) {}

export class B extends A.extend<B>("B")({
  a: Schema.Struct({
    x: Schema.String,
    y: Schema.String
  }),
  b: Schema.String
}) {}

Error: Duplicate property signature
details: Duplicate key "a"