Add datalog infrastructure for min aggregation function

This patch adds an instruction to the datalog interpreter and
constructs a new AST node for min aggregation functions.

The compiler is currently still work in progress and depends on
changes made to the handling of simple joins and the preprocessor.

Signed-off-by: Alex Horn <t-alexh@microsoft.com>

src/muz/base/dl_rule.h

@@ -346,6 +346,13 @@ namespace datalog {
 
         bool is_neg_tail(unsigned i) const { SASSERT(i < m_tail_size); return GET_TAG(m_tail[i]) == 1; }
 
+        /**
+           A predicate P(Xj) can be annotated by adding an interpreted predicate of the form ((_ min P N) ...)
+           where N is the column number that should be used for the min aggregation function.
+           Such an interpreted predicate is an example for which this function returns true.
+        */
+        bool is_min_tail(unsigned i) const { return dl_decl_plugin::is_aggregate(get_tail(i)->get_decl()); }
+
         /**
         Check whether predicate p is in the interpreted tail.
         

src/muz/base/dl_rule_set.h

@@ -179,6 +179,7 @@ namespace datalog {
         void compute_deps();
         void compute_tc_deps();
         bool stratified_negation();
+
     public:
         rule_set(context & ctx);
         rule_set(const rule_set & rs);