portability and compatibility fixes

compatibility-level

@@ -0,0 +1,27 @@
+o The `unset' builtin will unset the array a given an argument like `a[@]'.
+  Bash-5.2 will unset an element with key `@' (associative arrays) or remove
+  all the elements without unsetting the array (indexed arrays);
+
+o arithmetic commands ( ((...)) ) and the expressions in an arithmetic for
+  statement can be expanded more than once;
+
+o expressions used as arguments to arithmetic operators in the [[ conditional
+  command can be expanded more than once;
+
+o the expressions in substring parameter brace expansion can be
+  expanded more than once;
+
+o the expressions in the $(( ... )) word expansion can be expanded
+  more than once;
+
+o arithmetic expressions used as indexed array subscripts can be
+  expanded more than once;
+
+o `test -v', when given an argument of A[@], where A is an existing
+  associative array, will return true if the array has any set elements.
+  Bash-5.2 will look for a key named `@';
+
+o the ${param[:]=value} word expansion will return VALUE, before any
+  variable-specific transformations have been performed (e.g., converting
+  to lowercase). Bash-5.2 will return the final value assigned to the
+  variable.