diff --git a/README.md b/README.md
index f5ebd60..0f7027b 100644
--- a/README.md
+++ b/README.md
@@ -36,6 +36,7 @@
 ### Current task
 * [ ] Printable trait should take references
 ---
+* [ ] mutable parameters
 * [ ] migrate to pass-by-ref (branch `arc`)
 * [ ] Prefer candidates with mutable references, when possible
 * [ ] Fix problems with to_ir and loading references (especially globals). This causes issues in iterator
diff --git a/benchmarks/linalg/linalg/src/main.ppl b/benchmarks/linalg/linalg/src/main.ppl
index f25109b..fa2fada 100644
--- a/benchmarks/linalg/linalg/src/main.ppl
+++ b/benchmarks/linalg/linalg/src/main.ppl
@@ -151,21 +151,82 @@ fn <rows: Integer> x <columns: Integer> Hilbert Matrix -> Matrix:
 		i += 1
 	return m
 
+fn add <k: Rational> <a: RowSlice> to <b: MutRowSlice>:
+	assert a.matrix.columns == b.matrix.columns "Can't add rows of different lengths"
+	let n = a.matrix.columns
+	let mut i = 0
+	while i < n:
+		b[i] += k * a[i]
+		i += 1
+
+fn add <k: Rational> <a: MutRowSlice> to <b: MutRowSlice>:
+	add k (RowSlice from a) to b
+
+fn add <a: RowSlice> to <b: MutRowSlice>:
+	add 1.0 a to b
+
+fn add <a: MutRowSlice> to <b: MutRowSlice>:
+	add (RowSlice from a) to b
 
-fn gauss <a: Matrix> <b: Matrix> -> Matrix:
+fn det <a_: Matrix> -> Rational:
+	let mut a = a_
+	assert a.rows == a.columns "Matrix must be square"
+	let n = a.rows
+	let mut i = 0
+	while i < n:
+		let mut j = i + 1
+		while j < n:
+			let k = (a[j])[i] / (a[i])[i] // FIXME: division by zero
+			if k != 0.0:
+				add -k (a[i]) to (a[j])
+			j += 1
+		i += 1
+
+	let mut d = 1.0
+	i = 0
+	while i < n:
+		d *= (a[i])[i]
+		i += 1
+	return d
+
+
+fn gauss <a_: Matrix> <b_: Matrix> -> Matrix:
+	let mut a = a_
+	let mut b = b_
 	assert a.rows == a.columns "Matrix must be square"
 	assert a.rows == b.rows "Matrix and vector must have the same number of rows"
 	let n = a.rows
+	let mut i = 0
+	while i < n:
+		let mut j = i + 1
+		while j < n:
+			let k = (a[j])[i] / (a[i])[i] // FIXME: division by zero
+			if k != 0.0:
+				add -k (a[i]) to (a[j])
+				(b[j])[0] -= k * (b[i])[0]
+			j += 1
+		i += 1
+
+	i = n - 1
+	while i > 0:
+		let mut j = i - 1
+		while j >= 0:
+			let k = (a[j])[i] / (a[i])[i] // FIXME: division by zero
+			if k != 0.0:
+				add -k (a[i]) to (a[j])
+				(b[j])[0] -= k * (b[i])[0]
+			j -= 1
+		i -= 1
+
 	return b
 
 // let m = 3 x 3 Hilbert Matrix
 // print m
 
 fn run code:
-	let mut a = 1
-	let mut b = 2
-	swap a and b
-	print a
-	print b
+	let mut a = 3 x 3 Hilbert Matrix
+	println (det a)
+// let mut b = Vector from (repeat 1.0 3 times)
+// gauss a b 
 
 run code
\ No newline at end of file
diff --git a/ppl/src/math.ppl b/ppl/src/math.ppl
index 622c2ca..5ebea97 100644
--- a/ppl/src/math.ppl
+++ b/ppl/src/math.ppl
@@ -11,6 +11,17 @@ trait Add:
 		self = self + other
 //=================================
 
+//=================================
+//            Add
+//=================================
+/// Trait for `-` operation
+trait Sub:
+	fn <:Self> - <:Self> -> Self
+
+	fn <self: ReferenceMut<Self>> -= <other: Self>:
+		self = self - other
+//=================================
+
 
 //=================================
 //            Multiply