According to the user guide:

parens(x) ::= "(<x>)"
main() ::= "<["a", "b", "c"]:parens()>"

works and actually does fine. But the following doesn't:

parens(x, y) ::= "(<x>, <y>)"
main() ::= "<["a", "b", "c"], ["1", "2", "3"]:parens()>"

The error says parens requires i0 and i. So the following works:

parens(x, y, i, i0) ::= "(<x>, <y>)"
main() ::= "<["a", "b", "c"], ["1", "2", "3"]:parens()>"

The same works with anonymous template as follows:

parens(x, y) ::= "(<x>, <y>)"
main() ::= "<["a", "b", "c"], ["1", "2", "3"]:{x, y | parens(x, y) }>"

If i and i0 are intended to be available only in anonymous subtemplates (the doc seems suggesting so),
then at least the behavior must be consistent across non-zipping and zipping cases.

In non-zipping case, i and i0 aren't required and even if we specify them, no indices are passed in.
But in zipping case, if we add i and i0 to the argument list, the proper indices are passed in.

That's weird to me.

UPDATE: I read Interpreter.java and for the zip_map, if (st.impl.isAnonSubtemplate) check around st.rawSetAttribute("i0", i0) seems omitted by mistake. rot_map and rot_map_iterator have one.