Previously, we did not remember trusted external calls for later phase
when we compute possible verification targets for each function.
This led to false negative in cases where verification target can be
violated, but not by calling a public function directly, but only when
it is called as an external function from other function.
The added test cases witnesses this behaviour. The underflow in
`dec` cannot happen in any other way except what the `dec` is called
from `f`.
The same problem did not occur when the functions are called internally,
because for such cases, we have already been remembering these calls in
the callgraph in the CHC engine.
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