The C++ implementation of the fixedpoint engine (in
z3/src/api/api_datalog.cpp) already attempts to read `rlimit` from its
local parameters:
```c++
unsigned rlimit = to_fixedpoint(d)->m_params.get_uint("rlimit", mk_c(c)->get_rlimit());
```
However, because `rlimit` was not registered in the public fp parameter
schema (`fp_params.pyg`), any attempt by clients to set it locally via
`Z3_fixedpoint_set_params` was rejected by the Z3 parameter validator
with an "unknown parameter" error.
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