13.12 Nested multiple indexing in assignments
Stan interprets nested indexing in assingments as flat indexing so that a statement like
1] = b;a[:][
is the same as
1] = b;a[:,
However, this is inconsistent with multiple indexing rules. To avoid confusion nested multiple indexing in assignment will be an error after Stan 2.32. Nesting single indexing is still allowed as it cannot lead to ambiguity.
Scheduled Removal: Stan 2.32