Request - multiple types & index field

Would it be possible to have elements of multiple types with the same supertype, so that

• fields common to all structs in the array can be called via Array.Field.
  - the array can be filtered for given type via Array(::Type) (or similar syntax)
• If there is an "ID" field common to all structs, this can be used as a key.

abstract type MyType end

struct A<:MyType
    ID::String
    a::Int
end

struct B<:MyType
    ID::String
    b::Int
end

X = StructArray(      [ A("1",1), A("2",2), B("3",3), B("4",4) ],       parentType = MyType,      Key_Field = :ID   )


X.ID         # [1,2,3,4]

X["1"]       #  A("1",1)       select element by Key field (like a dictionary or named array)

X(::A)      # [A("1",1), A("2",2)]         filter for elements of type A

X(::A).a    # [1,2]                             reference field :a as it is common to elements of type A
X.a          #  Error                            not all elements contain this field

No files or tests are named. Start by reviewing the existing StructArrays indexing and element-type APIs, then compare them with the requested mixed-supertype, key-field, and type-filter operations; done means a decided API and working behavior for all examples.