How to mix in multiple schemas?

[adecusati]

I am working with Backstage. I have a schema for JSONSchema and for UISchema. Backstage templates use a schema that is basically jsonschema merged with uischema. How would i create a schema that works like this in KCL that includes validations from both the uischema and json schema modules?

It seems to me that i should be able to create two mixins that inherit these base schemas, but maybe i am misunderstanding something. If i define the following base schemas - with the idea that I cannot modify these directly (representing the json and ui schemas):

schema A:
  a?: str = "A"

schema B:
  b?: str = "B"

I create a third schema and mix these in as follows:

schema AMixin(A):

schema BMixin(B):

schema T:
  mixin [AMixin, BMixin]
  t?: str = "T"

If i try to create an object that uses T schema, I cannot set a or b or else i get "Cannot add member 'a' to schema 'T'". However, it works if i move the definition of a into AMixin directly, or if i instantiate AMixin instead of T. Should this not work? How else can this situation be handled? Ideally, I dont have to replicate the entire schema definition a second time...

[sanmai-NL]

@adecusati the grammar specifies the mixins must be multi-line:

//////////// mixin_stmt ////////////
mixin_stmt: "mixin" "[" [mixins | multiline_mixins] "]" "\n"
multiline_mixins: "\n" _INDENT mixins "\n" _DEDENT
mixins: operand_name ("," ("\n" mixins | operand_name))*