A pragma configures the compiler or engine from inside a program. Write it as a directive of the form #pragma key = value, and the setting applies when that program is compiled and run: 
#pragma prob_engine = mc.
#pragma prob_samples = 20000.
Pragmas keep engine choices next to the code they affect, so a program that needs Monte Carlo inference or a particular epistemic mode carries that requirement with it. When you pass the corresponding flag on the command line, the CLI flag overrides the pragma — the in-program directive is the default, and the invocation gets the final say.

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The ten pragmas

PragmaValuesDefault
prob_engineexact_ddnnf or mcexact_ddnnf
prob_cacheon or offnone
epistemic_modeg91 or faeelfaeel
prob_samples<int>10000
prob_seed<int>0
prob_confidence<float>0.95
prob_methodrejection or evidence_clampingnone
prob_max_nonmonotone_iterations<int> (must be > 0)1024
max_recursion_depth<int>1000
magic_setsauto, on, or offauto semantics
The probabilistic pragmas (prob_samples, prob_seed, prob_confidence, prob_method, prob_max_nonmonotone_iterations) shape Monte Carlo inference and take effect when prob_engine = mc. prob_max_nonmonotone_iterations is validated at parse time and must be strictly greater than zero.

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Magic sets

The magic_sets pragma controls a query-rewriting optimization. When a recursive query has bound arguments, magic sets rewrites the recursion to push those bindings inward so the engine derives only the facts the query can reach, rather than the whole relation. Setting on requests the rewrite and off disables it. The default, auto, applies the rewrite only when the compiler can prove the rewritten program is equivalent to the original; where it cannot establish that equivalence, auto declines and evaluates the program unchanged. This makes auto safe to leave on: it optimizes what it can prove and never risks changing your program’s meaning.