Add commands for forcing preferences during search

Add commands:

(prefer <formula>)
- will instruct case split queue to assign formula to true.
- prefer commands added within a scope are forgotten after leaving the scope.

(reset-preferences)
- resets asserted preferences. Has to be invoked at base level.

This provides functionality related to MathSAT and based on an ask by Tomáš Kolárik who is integrating the functionality with OpenSMT2

src/cmd_context/basic_cmds.cpp

@@ -334,6 +334,52 @@ public:
     }    
 };
 
+class prefer_cmd : public cmd {
+    expr *m_formula = nullptr;
+
+public:
+    prefer_cmd() : cmd("prefer") {}
+    char const *get_usage() const override {
+        return "<formula>";
+    }
+    char const *get_descr(cmd_context &ctx) const override {
+        return "set a preferred formula for the solver";
+    }
+    unsigned get_arity() const override {
+        return 1;
+    }
+    void prepare(cmd_context &ctx) override {
+        m_formula = nullptr;
+    }
+    cmd_arg_kind next_arg_kind(cmd_context &ctx) const override {
+        return CPK_EXPR;
+    }
+    void set_next_arg(cmd_context &ctx, expr *e) override {
+        m_formula = e;
+    }
+    void execute(cmd_context &ctx) override {
+        SASSERT(m_formula);
+        ctx.set_preferred(m_formula);
+    }
+};
+
+class reset_preferences_cmd : public cmd {
+    public:
+    reset_preferences_cmd() : cmd("reset-preferences") {}
+    char const *get_usage() const override {
+        return "";
+    }
+    char const *get_descr(cmd_context &ctx) const override {
+        return "reset all preferred formulas";
+    }
+    unsigned get_arity() const override {
+        return 0;
+    }
+    void execute(cmd_context &ctx) override {
+        ctx.reset_preferred();
+    }
+};
+
 class set_get_option_cmd : public cmd {
 protected:
     symbol      m_true;
@@ -926,6 +972,8 @@ void install_basic_cmds(cmd_context & ctx) {
     ctx.insert(alloc(get_info_cmd));
     ctx.insert(alloc(set_info_cmd));
     ctx.insert(alloc(set_initial_value_cmd));
+    ctx.insert(alloc(prefer_cmd));
+    ctx.insert(alloc(reset_preferences_cmd));
     ctx.insert(alloc(get_consequences_cmd));
     ctx.insert(alloc(builtin_cmd, "assert", "<term>", "assert term."));
     ctx.insert(alloc(builtin_cmd, "check-sat", "<boolean-constants>*", "check if the current context is satisfiable. If a list of boolean constants B is provided, then check if the current context is consistent with assigning every constant in B to true."));