diff --git a/convex-core/src/main/java/convex/core/data/AList.java b/convex-core/src/main/java/convex/core/data/AList.java
index 1b246706f..fa6c43508 100644
--- a/convex-core/src/main/java/convex/core/data/AList.java
+++ b/convex-core/src/main/java/convex/core/data/AList.java
@@ -59,7 +59,7 @@ public AList<T> empty() {
 	public abstract <R extends ACell> AList<R> map(Function<? super T, ? extends R> mapper);
 
 	@Override
-	public abstract <R extends ACell> AList<R> concat(ASequence<R> vals);
+	public abstract AList<T> concat(ASequence<? extends T> vals);
 
 	@Override
 	public abstract AList<T> assoc(long i, T value);
diff --git a/convex-core/src/main/java/convex/core/data/ASequence.java b/convex-core/src/main/java/convex/core/data/ASequence.java
index 1bc8f0653..fe12761cd 100644
--- a/convex-core/src/main/java/convex/core/data/ASequence.java
+++ b/convex-core/src/main/java/convex/core/data/ASequence.java
@@ -79,7 +79,7 @@ public <R extends ACell> ASequence<R> flatMap(Function<? super T, ? extends ASeq
 	 * @param vals A sequence of values to concatenate.
 	 * @return The concatenated sequence, of the same type as this sequence.
 	 */
-	public abstract <R extends ACell> ASequence<R> concat(ASequence<R> vals);
+	public abstract ASequence<T> concat(ASequence<? extends T> vals);
 
 	@Override
 	public final boolean addAll(int index, Collection<? extends T> c) {
diff --git a/convex-core/src/main/java/convex/core/data/AVector.java b/convex-core/src/main/java/convex/core/data/AVector.java
index 56b457961..c689133c3 100644
--- a/convex-core/src/main/java/convex/core/data/AVector.java
+++ b/convex-core/src/main/java/convex/core/data/AVector.java
@@ -140,7 +140,7 @@ public <R extends ACell> AVector<R> flatMap(Function<? super T, ? extends ASeque
 	}
 
 	@Override
-	public abstract <R extends ACell> AVector<R> concat(ASequence<R> b);
+	public abstract AVector<T> concat(ASequence<? extends T> b);
 
 	public abstract <R> R reduce(BiFunction<? super R, ? super T, ? extends R> func, R value);
 
@@ -212,11 +212,12 @@ public final <R extends ACell> AVector<R> conj(R value) {
 		return (AVector<R>) append((T) value);
 	}
 	
+	@SuppressWarnings("unchecked")
 	public <R extends ACell> AVector<R> conjAll(ACollection<R> xs) {
 		if (xs instanceof ASequence) {
-			return concat((ASequence<R>)xs);
+			return (AVector<R>) concat((ASequence<T>)xs);
 		}
-		return concat(Vectors.create(xs));
+		return (AVector<R>) concat(Vectors.create(xs));
 	}
 
 	@Override
diff --git a/convex-core/src/main/java/convex/core/data/List.java b/convex-core/src/main/java/convex/core/data/List.java
index 01b789254..a5ea85dfc 100644
--- a/convex-core/src/main/java/convex/core/data/List.java
+++ b/convex-core/src/main/java/convex/core/data/List.java
@@ -347,13 +347,13 @@ public <R extends ACell> AList<R> map(Function<? super T, ? extends R> mapper) {
 
 	@SuppressWarnings("unchecked")
 	@Override
-	public <R extends ACell> List<R> concat(ASequence<R> vals) {
+	public List<T> concat(ASequence<? extends T> vals) {
 		long n = RT.count(vals);
-		if (n==0) return (List<R>) this;
+		if (n==0) return this;
 
-		AVector<R> rvals;
+		AVector<T> rvals;
 		if (vals instanceof List) {
-			List<R> vlist=(List<R>) vals;
+			List<T> vlist=(List<T>) vals;
 			if (count==0) return vlist;
 			rvals = vlist.data;
 		} else {
@@ -363,7 +363,7 @@ public <R extends ACell> List<R> concat(ASequence<R> vals) {
 			}
 		}
 		if (count>0) {
-			rvals=rvals.concat((AVector<R>)data);
+			rvals=rvals.concat(data);
 		}
 		return List.reverse(rvals);
 	}
diff --git a/convex-core/src/main/java/convex/core/data/MapEntry.java b/convex-core/src/main/java/convex/core/data/MapEntry.java
index c35c46f6a..a6f876b7e 100644
--- a/convex-core/src/main/java/convex/core/data/MapEntry.java
+++ b/convex-core/src/main/java/convex/core/data/MapEntry.java
@@ -290,10 +290,9 @@ public void visitElementRefs(Consumer<Ref<ACell>> f) {
 		f.accept((Ref<ACell>) valueRef);
 	}
 
-	@SuppressWarnings("unchecked")
 	@Override
-	public <R  extends ACell> AVector<R> concat(ASequence<R> b) {
-		if (b.isEmpty()) return (AVector<R>) this;
+	public AVector<ACell> concat(ASequence<?> b) {
+		if (b.isEmpty()) return this;
 		return toVector().concat(b);
 	}
 
diff --git a/convex-core/src/main/java/convex/core/data/VectorArray.java b/convex-core/src/main/java/convex/core/data/VectorArray.java
index a2b859d1f..f75cb5512 100644
--- a/convex-core/src/main/java/convex/core/data/VectorArray.java
+++ b/convex-core/src/main/java/convex/core/data/VectorArray.java
@@ -151,7 +151,7 @@ public <R extends ACell> AVector<R> map(Function<? super T, ? extends R> mapper)
 	}
 
 	@Override
-	public <R extends ACell> AVector<R> concat(ASequence<R> b) {
+	public AVector<T> concat(ASequence<? extends T> b) {
 		return toVector().concat(b);
 	}
 
diff --git a/convex-core/src/main/java/convex/core/data/VectorLeaf.java b/convex-core/src/main/java/convex/core/data/VectorLeaf.java
index 2badc6c81..ccefd1df0 100644
--- a/convex-core/src/main/java/convex/core/data/VectorLeaf.java
+++ b/convex-core/src/main/java/convex/core/data/VectorLeaf.java
@@ -145,11 +145,11 @@ public AVector<T> append(T value) {
 
 	@SuppressWarnings("unchecked")
 	@Override
-	public <R extends ACell> AVector<R> concat(ASequence<R> b) {
+	public AVector<T> concat(ASequence<? extends T> b) {
 		// Maybe can optimise?
 		long aLen = count();
 		long bLen = b.count();
-		AVector<R> result = (AVector<R>) this;
+		AVector<T> result = this;
 		long i = aLen;
 		long end = aLen + bLen;
 		while (i < end) {
@@ -157,7 +157,7 @@ public <R extends ACell> AVector<R> concat(ASequence<R> b) {
 				int rn = Utils.checkedInt(Math.min(Vectors.CHUNK_SIZE, end - i));
 				if (rn == Vectors.CHUNK_SIZE) {
 					// we can append a whole chunk. Not toVector in case of VectorArrays
-					result = result.appendChunk((VectorLeaf<R>) b.subVector(i - aLen, rn).toVector());
+					result = result.appendChunk((AVector<T>) b.subVector(i - aLen, rn));
 					i += Vectors.CHUNK_SIZE;
 					continue;
 				}
diff --git a/convex-core/src/main/java/convex/core/data/VectorTree.java b/convex-core/src/main/java/convex/core/data/VectorTree.java
index a64cbe327..b07c8f0e5 100644
--- a/convex-core/src/main/java/convex/core/data/VectorTree.java
+++ b/convex-core/src/main/java/convex/core/data/VectorTree.java
@@ -293,21 +293,21 @@ public AVector<T> append(T value) {
 
 	@SuppressWarnings("unchecked")
 	@Override
-	public <R extends ACell> AVector<R> concat(ASequence<R> b) {
+	public AVector<T> concat(ASequence<? extends T> b) {
 		long bLen = b.count();
-		VectorTree<R> result = (VectorTree<R>) this;
+		VectorTree<T> result = this;
 		long bi = 0;
 		while (bi < bLen) {
 			if ((bi + Vectors.CHUNK_SIZE) <= bLen) {
 				// can append a whole chunk
-				AVector<R> chunk = b.subVector(bi, Vectors.CHUNK_SIZE);
+				AVector<T> chunk = (AVector<T>) b.subVector(bi, Vectors.CHUNK_SIZE);
 				result = result.appendChunk(chunk);
 				bi += Vectors.CHUNK_SIZE;
 			} else {
 				// we have less than a chunk left, so final result must be a VectorLeaf with the
 				// current result as tail
 				VectorLeaf<T> head = (VectorLeaf<T>) b.subVector(bi, bLen - bi).toVector();
-				return ((VectorLeaf<R>)head).withPrefix(result);
+				return ((VectorLeaf<T>)head).withPrefix(result);
 			}
 		}
 		return result;
diff --git a/convex-core/src/test/java/convex/core/data/EncodingSizeTest.java b/convex-core/src/test/java/convex/core/data/EncodingSizeTest.java
index 6676d6c9e..493780d9c 100644
--- a/convex-core/src/test/java/convex/core/data/EncodingSizeTest.java
+++ b/convex-core/src/test/java/convex/core/data/EncodingSizeTest.java
@@ -74,7 +74,7 @@ public static int size(ACell a) {
 	}
 	
 	@Test public void testVectorTree() {
-		AVector<?> a=Samples.MAX_EMBEDDED_VECTOR;
+		AVector<ACell> a=Samples.MAX_EMBEDDED_VECTOR;
 		for (int i=0; i<15; i++) {
 			a=a.concat(Samples.MAX_EMBEDDED_VECTOR);
 		}
diff --git a/convex-core/src/test/java/convex/core/data/EncodingTest.java b/convex-core/src/test/java/convex/core/data/EncodingTest.java
index 48938ee80..a6a7c4108 100644
--- a/convex-core/src/test/java/convex/core/data/EncodingTest.java
+++ b/convex-core/src/test/java/convex/core/data/EncodingTest.java
@@ -364,15 +364,15 @@ public void testFailedMissingEncoding() throws BadFormatException {
 		doMultiEncodingTest(Vectors.of(1,2,3));
 		
 		// Two non-embedded identical children
-		AVector<?> v1=Vectors.of(1,Samples.NON_EMBEDDED_STRING,Samples.NON_EMBEDDED_STRING,Samples.INT_VECTOR_23);
+		AVector<ACell> v1=Vectors.of(1,Samples.NON_EMBEDDED_STRING,Samples.NON_EMBEDDED_STRING,Samples.INT_VECTOR_23);
 		doMultiEncodingTest(v1);
 		
 		// Moar layers
-		AVector<?> v2=Vectors.of(7,Samples.NON_EMBEDDED_STRING,v1.concat(Samples.INT_VECTOR_23));
+		AVector<ACell> v2=Vectors.of(7,Samples.NON_EMBEDDED_STRING,v1.concat(Samples.INT_VECTOR_23));
 		doMultiEncodingTest(v2);
 		
 		// Mooooar layers
-		AVector<?> v3=Vectors.of(13,v2,v1,Samples.KEY_PAIR.signData(v2));
+		AVector<ACell> v3=Vectors.of(13,v2,v1,Samples.KEY_PAIR.signData(v2));
 		doMultiEncodingTest(v3);
 		
 		// Wrap in transaction