diff --git a/src/election.rs b/src/election.rs
index 66521fc..a99954f 100644
--- a/src/election.rs
+++ b/src/election.rs
@@ -450,7 +450,7 @@ impl<'a, N: Number> CountCard<'a, N> {
 	
 	/// Return the number of ballots across all parcels
 	pub fn num_ballots(&self) -> N {
-		return self.parcels.iter().fold(N::new(), |acc, p| acc + p.num_ballots());
+		return self.parcels.iter().fold(N::new(), |mut acc, p| { acc += p.num_ballots(); acc });
 	}
 }
 
@@ -468,7 +468,7 @@ pub struct Parcel<'a, N> {
 impl<'a, N: Number> Parcel<'a, N> {
 	/// Return the number of ballots in this parcel
 	pub fn num_ballots(&self) -> N {
-		return self.votes.iter().fold(N::new(), |acc, v| acc + &v.ballot.orig_value);
+		return self.votes.iter().fold(N::new(), |mut acc, v| { acc += &v.ballot.orig_value; acc });
 	}
 	
 	/// Return the value of the votes in this parcel
diff --git a/src/stv/gregory/mod.rs b/src/stv/gregory/mod.rs
index 288ce28..8fc2f98 100644
--- a/src/stv/gregory/mod.rs
+++ b/src/stv/gregory/mod.rs
@@ -82,7 +82,7 @@ where
 	
 	// Calculate loss by fraction - if minivoters used
 	if let Some(orig_total) = &state.election.total_votes {
-		let mut total_votes = state.candidates.values().fold(N::new(), |acc, cc| acc + &cc.votes);
+		let mut total_votes = state.candidates.values().fold(N::new(), |mut acc, cc| { acc += &cc.votes; acc });
 		total_votes += &state.exhausted.votes;
 		let lbf = orig_total - &total_votes;
 		
@@ -115,7 +115,7 @@ where
 	
 	if !has_surplus.is_empty() {
 		let total_surpluses = has_surplus.iter()
-			.fold(N::new(), |acc, c| acc + &state.candidates[c].votes - quota);
+			.fold(N::new(), |mut acc, c| { acc += &state.candidates[c].votes; acc -= quota; acc });
 		
 		// Determine if surplues can be deferred
 		if opts.defer_surpluses {
diff --git a/src/stv/gregory/transfers.rs b/src/stv/gregory/transfers.rs
index ce30032..2c14447 100644
--- a/src/stv/gregory/transfers.rs
+++ b/src/stv/gregory/transfers.rs
@@ -254,11 +254,16 @@ impl<'e, N: Number> TransferTable<'e, N> {
 				
 				// Sum total votes_out per candidate
 				for (candidate, cell) in self.total.cells.iter_mut() {
-					cell.votes_out = self.columns.iter().fold(N::new(), |acc, col| if let Some(cell) = col.cells.get(candidate) { acc + &cell.votes_out } else { acc });
+					cell.votes_out = self.columns.iter().fold(N::new(), |mut acc, col| {
+						if let Some(cell) = col.cells.get(candidate) {
+							acc += &cell.votes_out
+						}
+						acc
+					});
 				}
 				
 				// Sum total exhausted votes
-				self.total.exhausted.votes_out = self.columns.iter().fold(N::new(), |acc, col| acc + &col.exhausted.votes_out);
+				self.total.exhausted.votes_out = self.columns.iter().fold(N::new(), |mut acc, col| { acc += &col.exhausted.votes_out; acc });
 			}
 			RoundSubtransfersMode::PerBallot => {
 				// Calculate votes_out for each column
@@ -316,11 +321,16 @@ impl<'e, N: Number> TransferTable<'e, N> {
 				
 				// Sum total votes_out per candidate
 				for (candidate, cell) in self.total.cells.iter_mut() {
-					cell.votes_out = self.columns.iter().fold(N::new(), |acc, col| if let Some(cell) = col.cells.get(candidate) { acc + &cell.votes_out } else { acc });
+					cell.votes_out = self.columns.iter().fold(N::new(), |mut acc, col| {
+						if let Some(cell) = col.cells.get(candidate) {
+							acc += &cell.votes_out;
+						}
+						acc
+					});
 				}
 				
 				// Sum total exhausted votes
-				self.total.exhausted.votes_out = self.columns.iter().fold(N::new(), |acc, col| acc + &col.exhausted.votes_out);
+				self.total.exhausted.votes_out = self.columns.iter().fold(N::new(), |mut acc, col| { acc += &col.exhausted.votes_out; acc });
 			}
 		}
 	}
diff --git a/src/stv/meek.rs b/src/stv/meek.rs
index 5a8e21f..f73deba 100644
--- a/src/stv/meek.rs
+++ b/src/stv/meek.rs
@@ -130,7 +130,7 @@ where
 	
 	// Calculate loss by fraction - if minivoters used
 	if let Some(orig_total) = &state.election.total_votes {
-		let mut total_votes = state.candidates.values().fold(N::new(), |acc, cc| acc + &cc.votes);
+		let mut total_votes = state.candidates.values().fold(N::new(), |mut acc, cc| { acc += &cc.votes; acc });
 		total_votes += &state.exhausted.votes;
 		let lbf = orig_total - &total_votes;
 		
@@ -241,7 +241,7 @@ where
 		// Distribute if the total surplus exceeds the tolerance
 		let quota_tolerance = N::parse(&opts.meek_surplus_tolerance);
 		let total_surpluses = has_surplus.iter()
-			.fold(N::new(), |acc, c| acc + &state.candidates[c].votes - state.quota.as_ref().unwrap());
+			.fold(N::new(), |mut acc, c| { acc += &state.candidates[c].votes; acc -= state.quota.as_ref().unwrap(); acc });
 		return total_surpluses > quota_tolerance;
 	}
 }
@@ -266,7 +266,7 @@ where
 		// Determine if surplues can be deferred
 		if opts.defer_surpluses {
 			let total_surpluses = has_surplus.iter()
-				.fold(N::new(), |acc, c| acc + &state.candidates[c].votes - quota);
+				.fold(N::new(), |mut acc, c| { acc += &state.candidates[c].votes; acc -= quota; acc });
 			if super::can_defer_surpluses(state, opts, &total_surpluses) {
 				state.logger.log_literal(format!("Distribution of surpluses totalling {:.dps$} votes will be deferred.", total_surpluses, dps=opts.pp_decimals));
 				return Ok(false);
@@ -314,7 +314,7 @@ where
 			// Determine if surplues can be deferred
 			if should_distribute && opts.defer_surpluses {
 				let total_surpluses = has_surplus.iter()
-					.fold(N::new(), |acc, c| acc + &state.candidates[c].votes - quota);
+					.fold(N::new(), |mut acc, c| { acc += &state.candidates[c].votes; acc -= quota; acc });
 				if super::can_defer_surpluses(state, opts, &total_surpluses) {
 					surpluses_deferred = Some(total_surpluses);
 					break;
diff --git a/src/stv/mod.rs b/src/stv/mod.rs
index 7371097..e73ed09 100644
--- a/src/stv/mod.rs
+++ b/src/stv/mod.rs
@@ -1093,15 +1093,14 @@ fn elect_sure_winners<'a, N: Number>(state: &mut CountState<'a, N>, opts: &STVOp
 	
 	let mut total_trailing = N::new();
 	// For leading candidates, count only untransferred surpluses
-	total_trailing += hopefuls.iter().take(num_vacancies).fold(N::new(), |acc, (_, cc)| {
+	total_trailing += hopefuls.iter().take(num_vacancies).fold(N::new(), |mut acc, (_, cc)| {
 		if &cc.votes > state.quota.as_ref().unwrap() {
-			acc + &cc.votes - state.quota.as_ref().unwrap()
-		} else {
-			acc
+			acc += &cc.votes; acc -= state.quota.as_ref().unwrap();
 		}
+		acc
 	});
 	// For trailing candidates, count all votes
-	total_trailing += hopefuls.iter().skip(num_vacancies).fold(N::new(), |acc, (_, cc)| acc + &cc.votes);
+	total_trailing += hopefuls.iter().skip(num_vacancies).fold(N::new(), |mut acc, (_, cc)| { acc += &cc.votes; acc });
 	// Add finally any votes awaiting transfer
 	total_trailing += state.candidates.values().fold(N::zero(), |acc, cc| {
 		match cc.state {
@@ -1298,7 +1297,7 @@ where
 		let num_to_exclude = to_exclude.len();
 		if num_to_exclude > 0 {
 			let total_excluded = to_exclude.into_iter()
-				.fold(N::new(), |acc, c| acc + &state.candidates[c].votes);
+				.fold(N::new(), |mut acc, c| { acc += &state.candidates[c].votes; acc });
 			if total_surpluses >= &(&hopefuls[num_to_exclude].1.votes - &total_excluded) {
 				return false;
 			}
@@ -1497,7 +1496,7 @@ fn hopefuls_to_bulk_exclude<'a, N: Number>(state: &CountState<'a, N>, _opts: &ST
 	
 	let total_surpluses = state.candidates.iter()
 		.filter(|(_, cc)| !cc.finalised && &cc.votes > state.quota.as_ref().unwrap())
-		.fold(N::new(), |agg, (_, cc)| agg + &cc.votes - state.quota.as_ref().unwrap());
+		.fold(N::new(), |mut acc, (_, cc)| { acc += &cc.votes; acc -= state.quota.as_ref().unwrap(); acc });
 	
 	// Attempt to exclude as many candidates as possible
 	for i in 0..hopefuls.len() {
@@ -1509,7 +1508,7 @@ fn hopefuls_to_bulk_exclude<'a, N: Number>(state: &CountState<'a, N>, _opts: &ST
 		}
 		
 		// Do not exclude if this could change the order of exclusion
-		let total_votes = try_exclude.iter().fold(N::new(), |agg, (_, cc)| agg + &cc.votes);
+		let total_votes = try_exclude.iter().fold(N::new(), |mut acc, (_, cc)| { acc += &cc.votes; acc });
 		if i != 0 && total_votes + &total_surpluses >= hopefuls[hopefuls.len()-i].1.votes {
 			continue;
 		}
diff --git a/src/stv/sample.rs b/src/stv/sample.rs
index 68db912..065282a 100644
--- a/src/stv/sample.rs
+++ b/src/stv/sample.rs
@@ -186,7 +186,7 @@ where
 			
 			if opts.sample == SampleMethod::StratifyLR {
 				// Round remainders to remove loss by fraction
-				let transferred = candidate_transfers_remainders.values().fold(N::new(), |acc, (t, _)| acc + t);
+				let transferred = candidate_transfers_remainders.values().fold(N::new(), |mut acc, (t, _)| { acc += t; acc });
 				let loss_fraction = &surplus - &transferred;
 				if !loss_fraction.is_zero() && surplus_fraction.is_some() {
 					let n_to_round: usize = format!("{:.0}", loss_fraction).parse().expect("Loss by fraction overflows usize");