Can you give an example for which the proposed syntax would be problematic? Your example would be written as:

    let a = [: [: 1, 2, 3 :], [: 6, 7, 8 :] :]

or

    let a = [ [: 1, 2, 3 :], [: 6, 7, 8 :] ]

etc., depending on what mutability you want.