"Dereferencing" a key of a table

[meino.cramer@...]

Hi,

I have the following code via lua -i

    a={}
    a.bob="alice"
    keyname="bob"

    =a.keyname
    nil

Is there a way to access the key of an table, which name is store in
another variable?

Thank you very much in advance for any help!

Best regards,
mcc