Add option to choose solver

docs/smtchecker.rst

@@ -558,6 +558,39 @@ calls assume the called code is unknown and can do anything.
 The CHC engine is much more powerful than BMC in terms of what it can prove,
 and might require more computing resources.
 
+SMT and Horn solvers
+====================
+
+The two engines detailed above use automated theorem provers as their logical
+backends.  BMC uses an SMT solver, whereas CHC uses a Horn solver. Often the
+same tool can act as both, as seen in `z3 <https://github.com/Z3Prover/z3>`_,
+which is primarily an SMT solver and makes `Spacer
+<https://spacer.bitbucket.io/>`_ available as a Horn solver, and `Eldarica
+<https://github.com/uuverifiers/eldarica>`_ which does both.
+
+The user can choose which solvers should be used, if available, via the CLI
+option ``--model-checker-solvers {all,cvc4,smtlib2,z3}`` or the JSON option
+``settings.modelChecker.solvers=[smtlib2,z3]``, where:
+
+- ``cvc4`` is only available if the ``solc`` binary is compiled with it. Only BMC uses ``cvc4``.
+- ``smtlib2`` outputs SMT/Horn queries in the `smtlib2 <http://smtlib.cs.uiowa.edu/>`_ format.
+  These can be used together with the compiler's `callback mechanism <https://github.com/ethereum/solc-js>`_ so that
+  any solver binary from the system can be employed to synchronously return the results of the queries to the compiler.
+  This is currently the only way to use Eldarica, for example, since it does not have a C++ API.
+  This can be used by both BMC and CHC depending on which solvers are called.
+- ``z3`` is available
+
+  - if ``solc`` is compiled with it;
+  - if a dynamic ``z3`` library of version 4.8.x is installed in a Linux system (from Solidity 0.7.6);
+  - statically in ``soljson.js`` (from Solidity 0.6.9), that is, the Javascript binary of the compiler.
+
+Since both BMC and CHC use ``z3``, and ``z3`` is available in a greater variety
+of environments, including in the browser, most users will almost never need to be
+concerned about this option. More advanced users might apply this option to try
+alternative solvers on more complex problems.
+
+Please note that certain combinations of chosen engine and solver will lead to
+the SMTChecker doing nothing, for example choosing CHC and ``cvc4``.
 
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 Abstraction and False Positives