diff --git a/java/src/org/broadinstitute/sting/playground/gatk/walkers/reducereads/ConsensusSpan.java b/java/src/org/broadinstitute/sting/playground/gatk/walkers/reducereads/ConsensusSpan.java
index dd25915c9..aa32d43f6 100644
--- a/java/src/org/broadinstitute/sting/playground/gatk/walkers/reducereads/ConsensusSpan.java
+++ b/java/src/org/broadinstitute/sting/playground/gatk/walkers/reducereads/ConsensusSpan.java
@@ -1,5 +1,6 @@
 package org.broadinstitute.sting.playground.gatk.walkers.reducereads;
 
+import com.google.java.contract.*;
 import org.broadinstitute.sting.utils.GenomeLoc;
 
 /*
@@ -28,12 +29,14 @@ import org.broadinstitute.sting.utils.GenomeLoc;
  */
 
 /**
-* Created by IntelliJ IDEA.
-* User: depristo
-* Date: 4/8/11
-* Time: 3:01 PM
-* To change this template use File | Settings | File Templates.
-*/
+ * Created by IntelliJ IDEA.
+ * User: depristo
+ * Date: 4/8/11
+ * Time: 3:01 PM
+ *
+ * Represents a span of a consensus region (conserved, or variable) on the reference genome.  Supports
+ * either absolute or relative (refStart) positioning of the span.
+ */
 final class ConsensusSpan {
 
     /**
@@ -58,7 +61,13 @@ final class ConsensusSpan {
     final GenomeLoc loc;
     final Type consensusType;
 
+    @Requires({"refStart >= 0", "loc != null", "consensusType != null"})
+    @Ensures({"this.refStart == refStart", "this.loc.equals(loc)", "this.consensusType.equals(consensusType)"})
     public ConsensusSpan(final int refStart, GenomeLoc loc, ConsensusSpan.Type consensusType) {
+        if ( refStart < 0 ) throw new RuntimeException("RefStart must be greater than 0: " + refStart);
+        if ( loc == null ) throw new RuntimeException("Loc must not be null");
+        if ( consensusType == null ) throw new RuntimeException("ConsensusType must not be null");
+
         this.refStart = refStart;
         this.loc = loc;
         this.consensusType = consensusType;
@@ -68,10 +77,12 @@ final class ConsensusSpan {
         return loc.getStart() - refStart;
     }
 
+    @Ensures("result >= 0")
     public int getGenomeStart() {
         return loc.getStart();
     }
 
+    @Ensures("result >= 0")
     public int getGenomeStop() {
         return loc.getStop();
     }
@@ -80,13 +91,18 @@ final class ConsensusSpan {
         return consensusType;
     }
 
+    @Ensures("result >= 0")
     public int size() {
         return getGenomeStop() - getGenomeStart() + 1;
     }
 
+    @Ensures("result == !isVariable()")
     public boolean isConserved() { return getConsensusType() == Type.CONSERVED; }
+
+    @Ensures("result == !isConserved()")
     public boolean isVariable() { return getConsensusType() == Type.VARIABLE; }
 
+    @Ensures("result != null")
     public String toString() {
         return String.format("%s %s", consensusType, loc);
     }
diff --git a/java/test/org/broadinstitute/sting/playground/gatk/walkers/reducereads/ConsensusSpanUnitTest.java b/java/test/org/broadinstitute/sting/playground/gatk/walkers/reducereads/ConsensusSpanUnitTest.java
new file mode 100644
index 000000000..acbb481e0
--- /dev/null
+++ b/java/test/org/broadinstitute/sting/playground/gatk/walkers/reducereads/ConsensusSpanUnitTest.java
@@ -0,0 +1,150 @@
+// our package
+package org.broadinstitute.sting.playground.gatk.walkers.reducereads;
+
+
+// the imports for unit testing.
+
+
+import net.sf.picard.reference.IndexedFastaSequenceFile;
+import net.sf.samtools.SAMFileHeader;
+import net.sf.samtools.SAMRecord;
+import org.broadinstitute.sting.BaseTest;
+import org.broadinstitute.sting.gatk.walkers.qc.ValidateBAQWalker;
+import org.broadinstitute.sting.utils.GenomeLoc;
+import org.broadinstitute.sting.utils.GenomeLocParser;
+import org.broadinstitute.sting.utils.Utils;
+import org.broadinstitute.sting.utils.baq.BAQ;
+import org.broadinstitute.sting.utils.exceptions.UserException;
+import org.broadinstitute.sting.utils.sam.ArtificialSAMUtils;
+import org.testng.Assert;
+import org.testng.annotations.BeforeClass;
+import org.testng.annotations.BeforeMethod;
+import org.testng.annotations.DataProvider;
+import org.testng.annotations.Test;
+
+import java.io.File;
+import java.io.FileNotFoundException;
+import java.util.ArrayList;
+import java.util.List;
+
+/**
+ * Basic unit test for GenomeLoc
+ */
+public class ConsensusSpanUnitTest extends BaseTest {
+    File referenceFile = new File(hg19Reference);
+    GenomeLocParser genomeLocParser;
+    IndexedFastaSequenceFile fasta;
+    GenomeLoc loc;
+
+    @BeforeClass
+    public void before() {
+        try {
+            fasta = new IndexedFastaSequenceFile(referenceFile);
+            genomeLocParser = new GenomeLocParser(fasta.getSequenceDictionary());
+            loc = genomeLocParser.createGenomeLoc("1", 10, 19);
+
+        }
+        catch(FileNotFoundException ex) {
+            throw new UserException.CouldNotReadInputFile(referenceFile,ex);
+        }
+    }
+
+//    private class BAQTest {
+//        String readBases, refBases;
+//        byte[] quals, expected;
+//        String cigar;
+//        int refOffset;
+//        int pos;
+//
+//        public BAQTest(String _refBases, String _readBases, String _quals, String _expected) {
+//            this(0, -1, null, _readBases, _refBases, _quals, _expected);
+//        }
+//
+//        public BAQTest(int refOffset, String _refBases, String _readBases, String _quals, String _expected) {
+//            this(refOffset, -1, null, _refBases, _readBases, _quals, _expected);
+//        }
+//
+//        public BAQTest(long pos, String cigar, String _readBases, String _quals, String _expected) {
+//            this(0, pos, cigar, null, _readBases, _quals, _expected);
+//        }
+//
+//
+//        public BAQTest(int _refOffset, long _pos, String _cigar, String _refBases, String _readBases, String _quals, String _expected) {
+//            refOffset = _refOffset;
+//            pos = (int)_pos;
+//            cigar = _cigar;
+//            readBases = _readBases;
+//            refBases = _refBases;
+//
+//            quals = new byte[_quals.getBytes().length];
+//            expected = new byte[_quals.getBytes().length];
+//            for ( int i = 0; i < quals.length; i++) {
+//                quals[i] = (byte)(_quals.getBytes()[i] - 33);
+//                expected[i] = (byte)(_expected.getBytes()[i] - 33);
+//            }
+//        }
+//
+//        public String toString() { return readBases; }
+//
+//        public SAMRecord createRead() {
+//            SAMRecord read = ArtificialSAMUtils.createArtificialRead(header, "foo", 0, pos > 0 ? pos + (refOffset > 0 ? refOffset : 0): 1, readBases.getBytes(), quals);
+//            //if ( cigar != null ) read.setAlignmentEnd(readBases.getBytes().length + pos);
+//            read.setCigarString( cigar == null ? String.format("%dM", quals.length) : cigar);
+//            return read;
+//        }
+//    }
+
+    @Test(enabled = true)
+    public void testType() {
+        Assert.assertEquals(ConsensusSpan.Type.CONSERVED, ConsensusSpan.Type.otherType(ConsensusSpan.Type.VARIABLE));
+        Assert.assertEquals(ConsensusSpan.Type.VARIABLE, ConsensusSpan.Type.otherType(ConsensusSpan.Type.CONSERVED));
+    }
+
+    @Test(enabled = true)
+    public void testConsensusSpanOffset0() {
+        ConsensusSpan span = new ConsensusSpan(0, loc, ConsensusSpan.Type.CONSERVED);
+        Assert.assertEquals(span.getOffsetFromStartOfSites(), 10);
+        Assert.assertEquals(span.getGenomeStart(), loc.getStart());
+        Assert.assertEquals(span.getGenomeStop(), loc.getStop());
+        Assert.assertEquals(span.getConsensusType(), ConsensusSpan.Type.CONSERVED);
+        Assert.assertEquals(span.size(), 10);
+    }
+
+    @Test(enabled = true)
+    public void testConsensusSpanOffset10() {
+        ConsensusSpan span = new ConsensusSpan(10, loc, ConsensusSpan.Type.CONSERVED);
+        Assert.assertEquals(span.getOffsetFromStartOfSites(), 0);
+        Assert.assertEquals(span.getGenomeStart(), loc.getStart());
+        Assert.assertEquals(span.getGenomeStop(), loc.getStop());
+        Assert.assertEquals(span.getConsensusType(), ConsensusSpan.Type.CONSERVED);
+        Assert.assertEquals(span.size(), 10);
+    }
+
+    @Test(enabled = true)
+    public void testConsensusSpanTypes() {
+        ConsensusSpan conserved = new ConsensusSpan(0, loc, ConsensusSpan.Type.CONSERVED);
+        Assert.assertEquals(conserved.getConsensusType(), ConsensusSpan.Type.CONSERVED);
+        Assert.assertTrue(conserved.isConserved());
+        Assert.assertFalse(conserved.isVariable());
+
+        ConsensusSpan variable = new ConsensusSpan(0, loc, ConsensusSpan.Type.VARIABLE);
+        Assert.assertEquals(variable.getConsensusType(), ConsensusSpan.Type.VARIABLE);
+        Assert.assertFalse(variable.isConserved());
+        Assert.assertTrue(variable.isVariable());
+    }
+
+    @Test(enabled = true, expectedExceptions = {Error.class, Exception.class})
+    public void testBadSpanCreationBadOffset() {
+        ConsensusSpan span = new ConsensusSpan(-1, loc, ConsensusSpan.Type.CONSERVED);
+    }
+
+    @Test(enabled = true, expectedExceptions = {Error.class, Exception.class})
+    public void testBadSpanCreationNullLoc() {
+        ConsensusSpan span = new ConsensusSpan(0, null, ConsensusSpan.Type.CONSERVED);
+    }
+
+    @Test(enabled = true, expectedExceptions = {Error.class, Exception.class})
+    public void testBadSpanCreationNullType() {
+        ConsensusSpan span = new ConsensusSpan(0, loc, null);
+    }
+}
\ No newline at end of file