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thank you very much!

>> if we try to assign
>>   b[nil] = {}
>> then we get error 'table index is invalid', but if we try
>>    if b[nil] == nil then
>>       ...
>>    end
>> no error is raised. Is it correct behaviour?

AB> Yes.  The manual says:

>> An access to an indexed variable t[i] is equivalent to a
>> call gettable_event(t,i).

AB> and

>> An assignment to an indexed variable t[i] = val is
>> equivalent to settable_event(t,i,val).

AB> Both of these functions are

>> not defined or callable in Lua.  We use [them] here only
>> for explanatory purposes.

AB> Both gettable_event and settable_event use rawget to check
AB> the current value of the table, and rawget's description
AB> says:

>> index may be any value

AB> settable_event uses rawset to do the setting, and rawset's
AB> description says:

>> index [must be] any value different from nil

AB> See also <>.

Best regards,