Indexing Grammar in combination with vector construction using `[...]` causes ambiguous syntax.

Because `[[` can be used with a single number like `1` and `[` with a vector like `[1]`, it is not clear what the expression `x[[1]]` represents, i.e. does it call `[[` or `[`.

No implementation file or test is named in the issue. Start by reading the discussion and locating the parser or grammar entry points for indexing and vector construction, then reproduce x[[1]] and inspect how it is parsed. Done means the expression has an unambiguous, documented interpretation with coverage for the relevant syntax.

Written by the indexing model from the issue text.