diff --git a/src/main/java/net/alloyggp/griddle/validator/AnalyzedGame.java b/src/main/java/net/alloyggp/griddle/validator/AnalyzedGame.java
index dcbc76b..c737bfc 100644
--- a/src/main/java/net/alloyggp/griddle/validator/AnalyzedGame.java
+++ b/src/main/java/net/alloyggp/griddle/validator/AnalyzedGame.java
@@ -71,6 +71,9 @@ private static AnalyzedGame analyze(List<TopLevelGdl> rules) {
     private static Set<String> getNamesMatchingOrDownstreamOf(String target,
             Map<String, Set<String>> ancestorGraph) {
         Set<String> results = new HashSet<String>();
+        //Always add target, for cases like "true" or "does" that are not
+        //made true by sentences or rules within the GDL. (See GitHub issue #18)
+        results.add(target);
         for (String name : ancestorGraph.keySet()) {
             if (name.equalsIgnoreCase(target)) {
                 results.add(name);
diff --git a/src/main/java/net/alloyggp/griddle/validator/ValidatorConfiguration.java b/src/main/java/net/alloyggp/griddle/validator/ValidatorConfiguration.java
index 5d91f56..a59c581 100644
--- a/src/main/java/net/alloyggp/griddle/validator/ValidatorConfiguration.java
+++ b/src/main/java/net/alloyggp/griddle/validator/ValidatorConfiguration.java
@@ -11,6 +11,7 @@
 import net.alloyggp.griddle.validator.check.DatalogKeywordsNotConstantsCheck;
 import net.alloyggp.griddle.validator.check.DisjunctionNotNestedCheck;
 import net.alloyggp.griddle.validator.check.EmptyBodyCheck;
+import net.alloyggp.griddle.validator.check.EmptyDisjunctionCheck;
 import net.alloyggp.griddle.validator.check.ErrorStringCheck;
 import net.alloyggp.griddle.validator.check.FixedArityCheck;
 import net.alloyggp.griddle.validator.check.InconsistentCapitalizationCheck;
@@ -29,6 +30,7 @@
 import net.alloyggp.griddle.validator.check.TrueDoesAreNotStandaloneSentencesCheck;
 import net.alloyggp.griddle.validator.check.UnproducedSentenceNamesCheck;
 import net.alloyggp.griddle.validator.check.UnusedSentenceNamesCheck;
+import net.alloyggp.griddle.validator.check.VariablesOnlyInRulesCheck;
 
 public class ValidatorConfiguration {
     private final Map<Check, Level> checks;
@@ -46,6 +48,7 @@ public static ValidatorConfiguration createStandard() {
         checks.put(DisjunctionNotNestedCheck.INSTANCE, Level.WARNING);
         checks.put(ErrorStringCheck.INSTANCE, Level.ERROR);
         checks.put(EmptyBodyCheck.INSTANCE, Level.WARNING);
+        checks.put(EmptyDisjunctionCheck.INSTANCE, Level.WARNING);
         checks.put(FixedArityCheck.INSTANCE, Level.WARNING);
         checks.put(InconsistentCapitalizationCheck.INSTANCE, Level.ERROR);
         checks.put(InitBaseInputAreConstantCheck.INSTANCE, Level.ERROR);
@@ -63,6 +66,7 @@ public static ValidatorConfiguration createStandard() {
         checks.put(TrueDoesAreNotStandaloneSentencesCheck.INSTANCE, Level.ERROR);
         checks.put(UnproducedSentenceNamesCheck.INSTANCE, Level.WARNING);
         checks.put(UnusedSentenceNamesCheck.INSTANCE, Level.WARNING);
+        checks.put(VariablesOnlyInRulesCheck.INSTANCE, Level.ERROR);
 
         return new ValidatorConfiguration(checks);
     }
diff --git a/src/main/java/net/alloyggp/griddle/validator/check/EmptyDisjunctionCheck.java b/src/main/java/net/alloyggp/griddle/validator/check/EmptyDisjunctionCheck.java
new file mode 100644
index 0000000..d7773d8
--- /dev/null
+++ b/src/main/java/net/alloyggp/griddle/validator/check/EmptyDisjunctionCheck.java
@@ -0,0 +1,24 @@
+package net.alloyggp.griddle.validator.check;
+
+import net.alloyggp.griddle.grammar.GdlVisitor;
+import net.alloyggp.griddle.grammar.Literal;
+import net.alloyggp.griddle.validator.AnalyzedGame;
+
+public class EmptyDisjunctionCheck implements Check {
+    public static final EmptyDisjunctionCheck INSTANCE = new EmptyDisjunctionCheck();
+    private EmptyDisjunctionCheck() {
+        //Singleton
+    }
+
+    @Override
+    public void findProblems(AnalyzedGame game, final ProblemReporter reporter) {
+        game.visitAll(new GdlVisitor() {
+            @Override
+            public void visitDisjunction(Literal disjunction) {
+                if (disjunction.getDisjunction().isEmpty()) {
+                    reporter.report("Disjunctions should not be empty.", disjunction.getPosition());
+                }
+            }
+        });
+    }
+}
diff --git a/src/main/java/net/alloyggp/griddle/validator/check/VariablesOnlyInRulesCheck.java b/src/main/java/net/alloyggp/griddle/validator/check/VariablesOnlyInRulesCheck.java
new file mode 100644
index 0000000..b6c55d4
--- /dev/null
+++ b/src/main/java/net/alloyggp/griddle/validator/check/VariablesOnlyInRulesCheck.java
@@ -0,0 +1,30 @@
+package net.alloyggp.griddle.validator.check;
+
+import net.alloyggp.griddle.Position;
+import net.alloyggp.griddle.grammar.GdlVisitor;
+import net.alloyggp.griddle.grammar.Sentence;
+import net.alloyggp.griddle.grammar.TopLevelGdl;
+import net.alloyggp.griddle.validator.AnalyzedGame;
+
+public class VariablesOnlyInRulesCheck implements Check {
+    public static final VariablesOnlyInRulesCheck INSTANCE = new VariablesOnlyInRulesCheck();
+    private VariablesOnlyInRulesCheck() {
+        //Singleton
+    }
+
+    @Override
+    public void findProblems(AnalyzedGame game, final ProblemReporter reporter) {
+        for (TopLevelGdl gdl : game.getTopLevelComponents()) {
+            if (gdl.isSentence()) {
+                Sentence sentence = gdl.getSentence();
+                sentence.accept(new GdlVisitor() {
+                    @Override
+                    public void visitVariable(String variable, Position position) {
+                        reporter.report("Variables may only appear within rules.", position);
+                    }
+                });
+            }
+        }
+    }
+
+}
diff --git a/src/test/java/net/alloyggp/griddle/ValidatorTest.java b/src/test/java/net/alloyggp/griddle/ValidatorTest.java
index 7384718..d6798eb 100644
--- a/src/test/java/net/alloyggp/griddle/ValidatorTest.java
+++ b/src/test/java/net/alloyggp/griddle/ValidatorTest.java
@@ -5,6 +5,7 @@
 
 import net.alloyggp.griddle.validator.AnalyzedGame;
 import net.alloyggp.griddle.validator.ParenthesesValidator;
+import net.alloyggp.griddle.validator.Validators;
 import net.alloyggp.griddle.validator.check.Check;
 import net.alloyggp.griddle.validator.check.DatalogKeywordsNotConstantsCheck;
 import net.alloyggp.griddle.validator.check.ErrorStringCheck;
@@ -117,6 +118,36 @@ public void testMisplacedDatalogKeyword3() throws Exception {
         assertEquals(9, problem.getPosition().getEnd());
     }
 
+    @Test
+    public void testVariableInTopLevelSentence() throws Exception {
+        String gameString = TestGames.getGameString("varInTopLevelSentence");
+        Set<GdlProblem> problems = Validators.getStandardValidator().findProblems(gameString);
+        assertEquals(1, problems.size());
+        GdlProblem problem = problems.iterator().next();
+        assertEquals(10, problem.getPosition().getStart());
+        assertEquals(15, problem.getPosition().getEnd());
+    }
+
+    @Test
+    public void testEmptyDisjunction() throws Exception {
+        String gameString = TestGames.getGameString("emptyDisjunction");
+        Set<GdlProblem> problems = Validators.getStandardValidator().findProblems(gameString);
+        assertEquals(1, problems.size());
+        GdlProblem problem = problems.iterator().next();
+        assertEquals(31, problem.getPosition().getStart());
+        assertEquals(36, problem.getPosition().getEnd());
+    }
+
+    @Test
+    public void testBaseDependingOnTrue() throws Exception {
+        String gameString = TestGames.getGameString("baseDependsOnTrue");
+        Set<GdlProblem> problems = Validators.getStandardValidator().findProblems(gameString);
+        assertEquals(1, problems.size());
+        GdlProblem problem = problems.iterator().next();
+        assertEquals(15, problem.getPosition().getStart());
+        assertEquals(25, problem.getPosition().getEnd());
+    }
+
     private Set<GdlProblem> findProblems(Check check, String gameString) throws Exception {
         AnalyzedGame game = AnalyzedGame.parseAndAnalyze(gameString);
         final Set<GdlProblem> problems = new HashSet<GdlProblem>();