Re: help proving decidability of a set

This seems too hard. A is decidable if we can build a TM that always
halts and correctly says either "yes, the input is in A" or "no, the
input is not in A" It's not hard to build a machine that simulates in
parallel the transitions of two others running on the same input. Make
such a machine that simulates M1 and M2 in parallel on a given input X.
If the simulation of M1 eventually enumerates X, then the overall
machine halts and says "yes." If the simulation of M2 eventually
enumerates X, then the overall machine halts and says "no." By the
definitions of M1 and M2, exactly one of these events will occur for
each X, so this new composite machine has the desired property and
proves that A is decidable.

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