diff --git a/src/form/expression.cpp b/src/form/expression.cpp
index b6c944fc..683cda6e 100644
--- a/src/form/expression.cpp
+++ b/src/form/expression.cpp
@@ -128,7 +128,7 @@ bool Expression::contains(Type t, const std::string& name) const {
 
 size_t Expression::numTerms() const {
   size_t result = 1;
-  for (auto& c : children) {
+  for (const auto& c : children) {
     result += c.numTerms();
   }
   return result;
diff --git a/src/form/expression_util.cpp b/src/form/expression_util.cpp
index b86eb196..4cb6e235 100644
--- a/src/form/expression_util.cpp
+++ b/src/form/expression_util.cpp
@@ -124,9 +124,9 @@ bool pullUpChildren(Expression& e) {
   }
   Expression result(e.type, e.name, e.value);
   bool changed = false;
-  for (auto& c : e.children) {
+  for (const auto& c : e.children) {
     if (c.type == e.type) {
-      for (auto& d : c.children) {
+      for (const auto& d : c.children) {
         result.newChild(d);
       }
       changed = true;
@@ -157,10 +157,10 @@ bool multiplyThrough(Expression& e) {
   auto sum = e.children[1];       // copy
   e.type = Expression::Type::SUM;
   e.children.clear();
-  for (auto& c : sum.children) {
+  for (const auto& c : sum.children) {
     Expression prod(Expression::Type::PRODUCT, "", {constant});
     if (c.type == Expression::Type::PRODUCT) {
-      for (auto d : c.children) {
+      for (const auto& d : c.children) {
         prod.newChild(d);
       }
     } else {
@@ -326,7 +326,7 @@ void ExpressionUtil::collectNames(const Expression& e, Expression::Type type,
   if (e.type == type) {
     target.insert(e.name);
   }
-  for (auto& c : e.children) {
+  for (const auto& c : e.children) {
     collectNames(c, type, target);
   }
 }
@@ -348,14 +348,14 @@ Number ExpressionUtil::eval(const Expression& e,
     }
     case Expression::Type::SUM: {
       auto result = Number::ZERO;
-      for (auto c : e.children) {
+      for (const auto& c : e.children) {
         result = Semantics::add(result, eval(c, params));
       }
       return result;
     }
     case Expression::Type::PRODUCT: {
       auto result = Number::ONE;
-      for (auto c : e.children) {
+      for (const auto& c : e.children) {
         result = Semantics::mul(result, eval(c, params));
       }
       return result;
diff --git a/src/form/formula.cpp b/src/form/formula.cpp
index bde0a038..24bb9d55 100644
--- a/src/form/formula.cpp
+++ b/src/form/formula.cpp
@@ -108,7 +108,7 @@ std::multimap<std::string, std::string> Formula::getFunctionDeps(
   return deps;
 }
 
-bool Formula::isRecursive(std::string funcName) const {
+bool Formula::isRecursive(const std::string& funcName) const {
   auto deps = getFunctionDeps(false, false);
   for (auto it : deps) {
     if (it.first == funcName && it.second == funcName) {
@@ -158,7 +158,7 @@ void Formula::collectEntries(const Expression& e, Formula& target) {
   if (e.type == Expression::Type::FUNCTION && !e.name.empty()) {
     collectEntries(e.name, target);
   }
-  for (auto& c : e.children) {
+  for (const auto& c : e.children) {
     collectEntries(c, target);
   }
 }
diff --git a/src/form/formula.hpp b/src/form/formula.hpp
index 416d28f7..634b2262 100644
--- a/src/form/formula.hpp
+++ b/src/form/formula.hpp
@@ -28,7 +28,7 @@ class Formula {
   std::multimap<std::string, std::string> getFunctionDeps(
       bool transitive, bool ignoreSelf) const;
 
-  bool isRecursive(std::string funcName) const;
+  bool isRecursive(const std::string& funcName) const;
 
   void replaceAll(const Expression& from, const Expression& to);
 
diff --git a/src/form/formula_gen.cpp b/src/form/formula_gen.cpp
index ec314d42..aa88c28d 100644
--- a/src/form/formula_gen.cpp
+++ b/src/form/formula_gen.cpp
@@ -177,7 +177,7 @@ bool FormulaGenerator::update(const Operation& op) {
 }
 
 bool FormulaGenerator::update(const Program& p) {
-  for (auto& op : p.ops) {
+  for (const auto& op : p.ops) {
     if (!update(op)) {
       return false;  // operation not supported
     }
@@ -185,7 +185,7 @@ bool FormulaGenerator::update(const Program& p) {
   return true;
 }
 
-int64_t getNumInitialTermsNeeded(int64_t cell, const std::string fname,
+int64_t getNumInitialTermsNeeded(int64_t cell, const std::string& fname,
                                  const Formula& formula,
                                  const IncrementalEvaluator& ie) {
   auto stateful = ie.getStatefulCells();
@@ -406,7 +406,7 @@ void FormulaGenerator::simplifyFunctionNames() {
   }
   formula.replaceName(getCellName(0), canonicalName(0));
   size_t cell = 1;
-  for (auto& n : names) {
+  for (const auto& n : names) {
     if (n == getCellName(0)) {
       continue;
     }
@@ -419,7 +419,7 @@ void FormulaGenerator::simplifyFunctionNames() {
 bool addProgramIds(const Program& p, std::set<int64_t>& ids) {
   // TODO: check for recursion
   Parser parser;
-  for (auto op : p.ops) {
+  for (const auto& op : p.ops) {
     if (op.type == Operation::Type::SEQ) {
       auto id = op.source.value.asInt();
       if (ids.find(id) == ids.end()) {
diff --git a/src/form/pari.cpp b/src/form/pari.cpp
index 5431121f..b1c81ce6 100644
--- a/src/form/pari.cpp
+++ b/src/form/pari.cpp
@@ -55,7 +55,7 @@ void countFuncs(const Formula& f, const Expression& e,
       count[e]++;
     }
   }
-  for (auto& c : e.children) {
+  for (const auto& c : e.children) {
     countFuncs(f, c, count);
   }
 }
diff --git a/src/form/variant.cpp b/src/form/variant.cpp
index 64a4be9b..5103744a 100644
--- a/src/form/variant.cpp
+++ b/src/form/variant.cpp
@@ -7,13 +7,13 @@ VariantsManager::VariantsManager(
     const Formula& formula,
     const std::map<std::string, int64_t>& num_initial_terms) {
   // step 1: collect function names
-  for (auto& entry : formula.entries) {
+  for (const auto& entry : formula.entries) {
     if (ExpressionUtil::isSimpleFunction(entry.first, true)) {
       variants[entry.first.name] = {};
     }
   }
   // step 2: initialize function variants
-  for (auto& entry : formula.entries) {
+  for (const auto& entry : formula.entries) {
     if (ExpressionUtil::isSimpleFunction(entry.first, true)) {
       Variant variant;
       variant.func = entry.first.name;
@@ -81,7 +81,7 @@ void VariantsManager::collectFuncs(Variant& variant,
       variant.required_funcs.insert(expr.name);
     }
   }
-  for (auto& c : expr.children) {
+  for (const auto& c : expr.children) {
     collectFuncs(variant, c);
   }
 }
diff --git a/src/lang/analyzer.cpp b/src/lang/analyzer.cpp
index 31d51794..6dc6dec1 100644
--- a/src/lang/analyzer.cpp
+++ b/src/lang/analyzer.cpp
@@ -66,7 +66,7 @@ bool Analyzer::hasLogarithmicComplexity(const Program& program) {
   }
   // check updates of loop counter cell in loop body
   bool loop_counter_updated = false;
-  for (auto& op : simple_loop.body.ops) {
+  for (const auto& op : simple_loop.body.ops) {
     const auto target = op.target.value.asInt();
     if (target == simple_loop.counter) {
       // loop counter must be updated using division
@@ -104,7 +104,7 @@ bool isExponentialPreLoop(const Program& pre_loop, int64_t counter) {
     return false;
   }
   int64_t phase = 0;
-  for (auto& op : pre_loop.ops) {
+  for (const auto& op : pre_loop.ops) {
     auto target = op.target.value.asInt();
     // loop counter update
     if (target == counter) {
diff --git a/src/lang/comments.cpp b/src/lang/comments.cpp
index 417f5aca..782c87fe 100644
--- a/src/lang/comments.cpp
+++ b/src/lang/comments.cpp
@@ -54,7 +54,7 @@ std::string Comments::getSequenceIdFromProgram(const Program &p) {
   if (p.ops.empty()) {
     return id_str;  // not found
   }
-  auto &c = p.ops[0].comment;
+  const auto &c = p.ops[0].comment;
   if (c.length() > 1 && c[0] == 'A' && std::isdigit(c[1])) {
     id_str = c.substr(0, 2);
     for (size_t i = 2; i < c.length() && std::isdigit(c[i]); i++) {
diff --git a/src/lang/evaluator_inc.cpp b/src/lang/evaluator_inc.cpp
index 1118d1fd..a21aade1 100644
--- a/src/lang/evaluator_inc.cpp
+++ b/src/lang/evaluator_inc.cpp
@@ -126,7 +126,7 @@ bool IncrementalEvaluator::checkPreLoop(bool skip_input_transform) {
 bool IncrementalEvaluator::checkLoopBody() {
   // check loop counter cell
   bool loop_counter_updated = false;
-  for (auto& op : simple_loop.body.ops) {
+  for (const auto& op : simple_loop.body.ops) {
     const auto meta = Operation::Metadata::get(op.type);
     const auto target = op.target.value.asInt();
     if (target == simple_loop.counter) {
@@ -191,7 +191,7 @@ void IncrementalEvaluator::computeStatefulCells() {
   std::set<int64_t> read;
   std::set<int64_t> write;
   stateful_cells.clear();
-  for (auto& op : simple_loop.body.ops) {
+  for (const auto& op : simple_loop.body.ops) {
     const auto meta = Operation::Metadata::get(op.type);
     if (meta.num_operands == 0) {
       continue;
@@ -222,7 +222,7 @@ void IncrementalEvaluator::computeLoopCounterDependentCells() {
   bool changed = true;
   while (changed) {
     changed = false;
-    for (auto& op : simple_loop.body.ops) {
+    for (const auto& op : simple_loop.body.ops) {
       const auto meta = Operation::Metadata::get(op.type);
       const auto target = op.target.value.asInt();
       if (loop_counter_dependent_cells.find(target) !=
@@ -253,7 +253,7 @@ bool IncrementalEvaluator::checkPostLoop() {
   // initialize output cells. all memory cells that are read
   // by the post-loop fragment are output cells.
   std::set<int64_t> write;
-  for (auto& op : simple_loop.post_loop.ops) {
+  for (const auto& op : simple_loop.post_loop.ops) {
     const auto meta = Operation::Metadata::get(op.type);
     if (meta.num_operands < 1) {
       continue;