diff --git a/passes/techmap/csa_tree.cc b/passes/techmap/csa_tree.cc
index 323fd6818..110575a0e 100644
--- a/passes/techmap/csa_tree.cc
+++ b/passes/techmap/csa_tree.cc
@@ -1,5 +1,10 @@
 /**
  * Replaces chains of $add/$sub and $macc cells with carry-save adder trees
+ *
+ * Terminology:
+ * - parent:    Cells that consume another cell's output
+ * - chainable: Adds/subs with no carry-out usage
+ * - chain:     Connected path of chainable cells
  */
 
 #include "kernel/macc.h"
@@ -83,8 +88,6 @@ struct AluInfo {
 		return GetSize(bi) == 1 && bi[0] == State::S0 && GetSize(ci) == 1 && ci[0] == State::S0;
 	}
 
-	// Chainable cells are adds/subs with no carry usage, connected chainable
-	// cells form chains that can be replaced with CSA trees.
 	bool is_chainable(Cell *cell)
 	{
 		if (!(is_add(cell) || is_subtract(cell)))
@@ -130,7 +133,6 @@ struct Rewriter {
 		return consumer;
 	}
 
-	// Find cells that consume another cell's output.
 	dict<Cell *, Cell *> find_parents(const pool<Cell *> &candidates)
 	{
 		dict<Cell *, Cell *> parent_of;
@@ -201,6 +203,7 @@ struct Rewriter {
 		if (!parent_subtracts)
 			return false;
 
+		// Check if any bit of child's Y connects to parent's B
 		SigSpec child_y = traversal.sigmap(child->getPort(ID::Y));
 		SigSpec parent_b = traversal.sigmap(parent->getPort(ID::B));
 		for (auto bit : child_y)
@@ -214,6 +217,8 @@ struct Rewriter {
 						    int &neg_compensation)
 	{
 		pool<SigBit> chain_bits = internal_bits(chain);
+
+		// Propagate negation flags through chain
 		dict<Cell *, bool> negated;
 		negated[root] = false;
 		{
@@ -233,15 +238,12 @@ struct Rewriter {
 			}
 		}
 
+		// Extract leaf operands
 		std::vector<Operand> operands;
 		neg_compensation = 0;
 
 		for (auto cell : chain) {
-			bool cell_neg;
-			if (negated.count(cell))
-				cell_neg = negated[cell];
-			else
-				cell_neg = false;
+			bool cell_neg = negated.count(cell) ? negated[cell] : false;
 
 			SigSpec a = traversal.sigmap(cell->getPort(ID::A));
 			SigSpec b = traversal.sigmap(cell->getPort(ID::B));
@@ -249,10 +251,10 @@ struct Rewriter {
 			bool b_signed = cell->getParam(ID::B_SIGNED).as_bool();
 			bool b_sub = (cell->type == ID($sub)) || (cells.is_alu(cell) && alu_info.is_subtract(cell));
 
+			// Only add operands not produced by other chain cells
 			if (!overlaps(a, chain_bits)) {
-				bool neg = cell_neg;
-				operands.push_back({a, a_signed, neg});
-				if (neg)
+				operands.push_back({a, a_signed, cell_neg});
+				if (cell_neg)
 					neg_compensation++;
 			}
 			if (!overlaps(b, chain_bits)) {