* each account must have an appropriate signer, if there is no signer for
this account in the tx it's an error
* we can only safely append to Scripts when account belongs to the next
signer (we don't have appropriate verification scripts for other signers)
* when contract has one parameter, the signature shouldn't be appended to
other data
I think these rules allow to handle more cases and do that safer. We have more
complex scenarios though, like non-signature parameters or mixed-parameter
invocation scripts, but that's out of scope for now.
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