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sclass LogicRuleApplier {
  BasicLogicRule rule;
  O fact;
  
  // define what a variable is
  swappable bool isVar(O o) { ret o instanceof S && isDollarVar(o/S); }

  // where does your output go
  swappable void addRewrittenRHS(O rhs, O trail) {
    print("Derived: " + rhs);
  }
  
  // use standard deep-zip & replace utils
  swappable Map deepZip(O a, O b) { ret newGazelle_deepZip1(a, b); }
  swapapble O replaceVars(O o, Map map) { ret replaceVars_understandsTransformableAndList(o, map); }

  run {
    // now we match the fact to our condition
    Map map = deepZip(rule.lhs, fact);

    // Discard unless all keys are variables
    if (map != null && !all(keys(map), s -> isVar(s))) map = null;
    
    if (map == null)
      ret with if (printNonMatches) print("Non-match: " + quote(cond) + " / " + quote(fact));

    // Now we have a proper mapping with the keys being variables!
    if (verbose) print("Match: " + quote(cond) + " / " + quote(fact));

    // Apply mapping to right hand side
    O rhs_replaced = replaceVars(rule.rhs, map);
    if (verbose) print(+rhs_replaced);
    
    Map trail = litorderedmap(op := "applyLogicRuleToFact",
      +rule, +fact, +rhs_replaced);

    addRewrittenRHS(rhs_replaced, trail);
  }
}