diff --git a/code/query/expression.hxx b/code/query/expression.hxx
index 73adad3..2ab8b78 100644
--- a/code/query/expression.hxx
+++ b/code/query/expression.hxx
@@ -23,11 +23,11 @@ public:
 
   friend
   void
-  accept(expression_t const& e, visitor_t& v);
+  accept(expression_t const&, visitor_t&);
 
   friend
   string
-  to_string(expression_t const& e);
+  to_string(expression_t const&);
 
 private:
   struct abstract_t
diff --git a/code/query/logical-and.hxx b/code/query/logical-and.hxx
index 7180c78..6088e6f 100644
--- a/code/query/logical-and.hxx
+++ b/code/query/logical-and.hxx
@@ -32,7 +32,7 @@ namespace code::query
   };
 
   string
-  to_string(logical_and_t const);
+  to_string(logical_and_t const&);
 
 } // namespace code::query
 
diff --git a/code/query/logical-or.hxx b/code/query/logical-or.hxx
index 603f3ba..2e8cf7d 100644
--- a/code/query/logical-or.hxx
+++ b/code/query/logical-or.hxx
@@ -32,7 +32,7 @@ namespace code::query
   };
 
   string
-  to_string(logical_or_t const);
+  to_string(logical_or_t const&);
 
 } // namespace code::query
 
diff --git a/code/query/match.cxx b/code/query/match.cxx
index c9b6bcc..b999647 100644
--- a/code/query/match.cxx
+++ b/code/query/match.cxx
@@ -25,10 +25,14 @@ namespace code::query
     public:
       static
       bool
-      match(expression_t const& e, predicate_t const& p)
+      match(optional<expression_t> const& e, predicate_t const& p)
       {
         matcher_t m{p};
-        accept(e, m);
+
+        if (e) {
+          accept(*e, m);
+        }
+
         return m.result;
       }
 
diff --git a/code/query/parse.cxx b/code/query/parse.cxx
index beb2a10..411951a 100644
--- a/code/query/parse.cxx
+++ b/code/query/parse.cxx
@@ -6,11 +6,10 @@
 namespace code::query
 {
 
-  optional<query_t>
+  query_t
   try_parse(string const& q, parse_context_t& context)
   {
     string_lexical_analyzer_t lexer{q.begin(), q.end()};
-
     return syntactical_analyzer_t{lexer, context}.try_parse();
   }
 
diff --git a/code/query/parse.hxx b/code/query/parse.hxx
index bab6f99..c956377 100644
--- a/code/query/parse.hxx
+++ b/code/query/parse.hxx
@@ -8,7 +8,7 @@
 namespace code::query
 {
 
-  optional<query_t>
+  query_t
   try_parse(string const&, parse_context_t&);
 
 } // namespace code::query
diff --git a/code/query/query.cxx b/code/query/query.cxx
index 178ba87..666bb33 100644
--- a/code/query/query.cxx
+++ b/code/query/query.cxx
@@ -4,7 +4,7 @@ namespace code::query
 {
 
   query_t::
-  query_t(expression_t e,
+  query_t(optional<expression_t> e,
           vector<warning_t> warnings,
           vector<error_t> errors)
     : expr_{move(e)},
@@ -12,7 +12,7 @@ namespace code::query
       errors_{move(errors)}
   {}
 
-  expression_t const&
+  optional<expression_t> const&
   query_t::
   expr() const
   {
@@ -36,13 +36,19 @@ namespace code::query
   void
   accept(query_t const& q, visitor_t& v)
   {
-    accept(q.expr(), v);
+    if (q.expr()) {
+      accept(*q.expr(), v);
+    }
   }
 
   string
   to_string(query_t const& q)
   {
-    return to_string(q.expr());
+    if (q.expr()) {
+      return to_string(*q.expr());
+    }
+
+    return string{};
   }
 
 } // namespace code::query
diff --git a/code/query/query.hxx b/code/query/query.hxx
index ff67d4c..77b4c89 100644
--- a/code/query/query.hxx
+++ b/code/query/query.hxx
@@ -12,11 +12,11 @@ namespace code::query
   class query_t
   {
   public:
-    query_t(expression_t e,
+    query_t(optional<expression_t> e,
             vector<warning_t> warnings,
             vector<error_t> errors);
 
-    expression_t const&
+    optional<expression_t> const&
     expr() const;
 
     vector<warning_t> const&
@@ -26,7 +26,7 @@ namespace code::query
     errors() const;
 
   private:
-    expression_t expr_;
+    optional<expression_t> expr_;
     vector<warning_t> warnings_;
     vector<error_t> errors_;
 
diff --git a/code/query/syntactical-analyzer.cxx b/code/query/syntactical-analyzer.cxx
index 8ce2ec0..ffce29a 100644
--- a/code/query/syntactical-analyzer.cxx
+++ b/code/query/syntactical-analyzer.cxx
@@ -31,7 +31,7 @@ namespace code::query
     return context_;
   }
 
-  optional<query_t>
+  query_t
   syntactical_analyzer_t::
   try_parse()
   {
@@ -43,15 +43,13 @@ namespace code::query
       if (last.type() != token_type_t::end) {
         context().report_warning({{}, "trailing token at end of query"});
       }
-
-      return query_t{
-        *move(expr),
-        context().warnings(),
-        context().errors()
-      };
     }
 
-    return nullopt;
+    return query_t{
+      move(expr),
+      context().warnings(),
+      context().errors()
+    };
   }
 
   optional<expression_t>
@@ -63,7 +61,11 @@ namespace code::query
     for (;;) {
       auto t = lexer().peek();
 
-      if (t.type() == token_type_t::logical_not) {
+      if (t.type() == token_type_t::end) {
+        break;
+      }
+
+      else if (t.type() == token_type_t::logical_not) {
         lexer().consume();
 
         auto rhs = try_parse_primary_expression();
@@ -75,7 +77,7 @@ namespace code::query
 
         if (expr) {
           expr = logical_and_t{
-            source_location_t{}, *expr, logical_not_t{{}, *rhs}
+            {}, *expr, logical_not_t{{}, *rhs}
           };
         }
         else {
@@ -91,6 +93,12 @@ namespace code::query
         if (lexer().peek().type() == token_type_t::colon) {
           lexer().consume();
 
+          // consume trailing colons as well.
+          //
+          while (lexer().peek().type() == token_type_t::colon) {
+            lexer().consume();
+          }
+
           auto identifier = *t.value();
 
           // Next expect a term or quoted term.
@@ -100,7 +108,8 @@ namespace code::query
           if (t.type() != token_type_t::simple_term &&
               t.type() != token_type_t::quoted_term) {
             context().report_warning({{}, "expected simple-term or quoted-term after tag" });
-            break;
+            lexer().consume();
+            continue;
           }
 
           lexer().consume();
@@ -114,7 +123,7 @@ namespace code::query
           };
 
           if (expr) {
-            expr = logical_and_t{
+            expr = logical_or_t{
               {}, *expr, tag_t{{}, move(identifier), move(term)}
             };
           }
@@ -123,7 +132,7 @@ namespace code::query
           }
         }
         else if (expr) {
-          expr = logical_and_t{
+          expr = logical_or_t{
             {}, *expr, term_t{{}, term_t::simple, *t.value()}
           };
         }
@@ -134,7 +143,7 @@ namespace code::query
 
       else if (t.type() == token_type_t::quoted_term) {
         if (expr) {
-          expr = logical_and_t{
+          expr = logical_or_t{
             {}, *expr, term_t{{}, term_t::quoted, *t.value()}
           };
         }
@@ -159,23 +168,24 @@ namespace code::query
 
         if (!next_expr) {
           context().report_warning({{}, "expected expression inside parenthesis"});
-          break;
+          lexer().consume();
+          continue;
         }
 
         next_expr = parenthesized_t{{}, *next_expr};
 
         if (expr) {
-          expr = logical_and_t{{}, *expr, *next_expr};
+          expr = logical_or_t{{}, *expr, *next_expr};
         }
         else {
           expr = next_expr;
         }
 
         t = lexer().peek();
-
         if (t.type() != token_type_t::close_parens) {
           context().report_warning({{}, "expected end parenthesis"});
-          break;
+          lexer().consume();
+          continue;
         }
 
         lexer().consume();
@@ -195,8 +205,9 @@ namespace code::query
   {
     auto lhs = try_parse_primary_expression();
 
-    if (!lhs)
+    if (!lhs) {
       return nullopt;
+    }
 
     while (lexer().peek().type() == token_type_t::logical_and) {
       lexer().consume();
@@ -239,6 +250,34 @@ namespace code::query
   syntactical_analyzer_t::
   try_parse_expression()
   {
+    auto invalid =  [](token_t const& token)
+    {
+      if (token.type() == token_type_t::end) {
+        return false;
+      }
+
+      if (token.type() == token_type_t::simple_term) {
+        return false;
+      }
+
+      if (token.type() == token_type_t::quoted_term) {
+        return false;
+      }
+
+      if (token.type() == token_type_t::open_parens) {
+        return false;
+      }
+
+      return true;
+    };
+
+    // filter leading invalid tokens.
+    //
+    while (invalid(lexer().peek())) {
+      context().report_warning({{}, "invalid leading token"});
+      lexer().consume();
+    }
+
     return try_parse_logical_or();
   }
 
diff --git a/code/query/syntactical-analyzer.hxx b/code/query/syntactical-analyzer.hxx
index b4d7c11..76162d8 100644
--- a/code/query/syntactical-analyzer.hxx
+++ b/code/query/syntactical-analyzer.hxx
@@ -21,7 +21,7 @@ namespace code::query
     parse_context_t&
     context();
 
-    optional<query_t>
+    query_t
     try_parse();
 
   private:
diff --git a/code/query/token.cxx b/code/query/token.cxx
index bd3d90f..00ae986 100644
--- a/code/query/token.cxx
+++ b/code/query/token.cxx
@@ -23,4 +23,56 @@ namespace code::query
     return value_;
   }
 
+  std::ostream&
+  operator<<(std::ostream& o, token_t const& token)
+  {
+    switch (token.type()) {
+      case token_type_t::end:
+        o << "end";
+        break;
+
+      case token_type_t::simple_term:
+        o << "simple-term";
+        break;
+
+      case token_type_t::quoted_term:
+        o << "quoted-term";
+        break;
+
+      case token_type_t::colon:
+        o << "colon";
+        break;
+
+      case token_type_t::logical_and:
+        o << "logical-and";
+        break;
+
+      case token_type_t::logical_not:
+        o << "logical-not";
+        break;
+
+      case token_type_t::logical_or:
+        o << "logical-or";
+        break;
+
+      case token_type_t::open_parens:
+        o << "open-parens";
+        break;
+
+      case token_type_t::close_parens:
+        o << "close-parens";
+        break;
+
+
+    }
+
+    if (auto v = token.value(); v) {
+      o << ": " << *v;
+    }
+
+    o << '\n';
+
+    return o;
+  }
+
 } // namespace code::query
diff --git a/code/query/token.hxx b/code/query/token.hxx
index 403488d..ba1eb55 100644
--- a/code/query/token.hxx
+++ b/code/query/token.hxx
@@ -3,6 +3,8 @@
 
 #include <code/query/types.hxx>
 
+#include <iostream>
+
 namespace code::query
 {
 
@@ -42,6 +44,9 @@ namespace code::query
 
   };
 
+  std::ostream&
+  operator<<(std::ostream&, token_t const&);
+
 } // namespace code::query
 
 #endif