The version number decides if opcodes are 1 or 2 bytes, but we don't
know for sure which it is until the first opcode has been handled. But
the way the o_versionOp1 opcode is defined, we should recognize it it
regardless, so going from assumed version 2 to confirmed version 1
should be no problem. I do not have any test case to confirm this,
though.
What I do know is that going from assumed version 1 to confirmed version
2 does not work, so we want _version to be well defined (even if wrong)
from the beginning.
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