Use const refs to reduce copying.

These are things that have been found by `clang-tidy`.

src/muz/spacer/spacer_context.cpp

@@ -3418,7 +3418,7 @@ expr_ref context::get_constraints (unsigned level)
     return m_pm.mk_and (constraints);
 }
 
-void context::add_constraints (unsigned level, expr_ref c)
+void context::add_constraints (unsigned level, const expr_ref& c)
 {
     if (!c.get()) { return; }
     if (m.is_true(c)) { return; }

src/muz/spacer/spacer_context.h

@@ -833,7 +833,7 @@ public:
     pob& get_root() const { return m_pob_queue.get_root(); }
 
     expr_ref get_constraints (unsigned lvl);
-    void add_constraints (unsigned lvl, expr_ref c);
+    void add_constraints (unsigned lvl, const expr_ref& c);
 };
 
 inline bool pred_transformer::use_native_mbp () {return ctx.use_native_mbp ();}

src/muz/spacer/spacer_matrix.cpp

@@ -42,7 +42,7 @@ namespace spacer
         return m_num_cols;
     }
 
-    rational spacer_matrix::get(unsigned int i, unsigned int j)
+    const rational& spacer_matrix::get(unsigned int i, unsigned int j)
     {
         SASSERT(i < m_num_rows);
         SASSERT(j < m_num_cols);
@@ -50,7 +50,7 @@ namespace spacer
         return m_matrix[i][j];
     }
 
-    void spacer_matrix::set(unsigned int i, unsigned int j, rational v)
+    void spacer_matrix::set(unsigned int i, unsigned int j, const rational& v)
     {
         SASSERT(i < m_num_rows);
         SASSERT(j < m_num_cols);

src/muz/spacer/spacer_matrix.h

@@ -30,8 +30,8 @@ namespace spacer {
         unsigned num_rows();
         unsigned num_cols();
 
-        rational get(unsigned i, unsigned j);
-        void set(unsigned i, unsigned j, rational v);
+        const rational& get(unsigned i, unsigned j);
+        void set(unsigned i, unsigned j, const rational& v);
 
         unsigned perform_gaussian_elimination();
 

src/muz/spacer/spacer_unsat_core_learner.cpp

@@ -301,7 +301,7 @@ public:
     void operator()(quantifier*) {}
 };
 
-void unsat_core_learner::collect_symbols_b(expr_set axioms_b)
+void unsat_core_learner::collect_symbols_b(const expr_set& axioms_b)
 {
     expr_mark visited;
     collect_pure_proc proc(m_symbols_b);

src/muz/spacer/spacer_unsat_core_learner.h

@@ -82,7 +82,7 @@ namespace spacer {
     private:
         ptr_vector<unsat_core_plugin> m_plugins;
         func_decl_set m_symbols_b; // symbols, which occur in any b-asserted formula
-        void collect_symbols_b(expr_set axioms_b);
+        void collect_symbols_b(const expr_set& axioms_b);
 
         ast_mark m_a_mark;
         ast_mark m_b_mark;