Question

Allows a program to traverse all fields of a table.
Its first argument is a table and its second argument
is an index in this table.
next returns the next index of the table
and its associated value.
When called with nil as its second argument,
next returns an initial index
and its associated value.
When called with the last index,
or with nil in an empty table,
next returns nil.
If the second argument is absent, then it is interpreted as nil.
In particular,
you can use next(t) to check whether a table is empty.

The order in which the indices are enumerated is not specified,
even for numeric indices.
(To traverse a table in numeric order,
use a numerical for or the ipairs function.)

The behavior of next is undefined if,
during the traversal,
you assign any value to a non-existent field in the table.
You may however modify existing fields.
In particular, you may clear existing fields.