diff --git a/src/interval_set.rs b/src/interval_set.rs
index acc6a74..111c3b1 100644
--- a/src/interval_set.rs
+++ b/src/interval_set.rs
@@ -1,5 +1,4 @@
 use super::*;
-use std::convert::From;
 use std::ops::{Add, BitAnd, BitOr, Deref, DerefMut, Not, Sub};
 
 /// A coalescing set of intervals
@@ -159,43 +158,41 @@ impl From<&[Interval]> for IntervalSet {
     }
 }
 
-impl Not for &IntervalSet {
-    type Output = IntervalSet;
-    fn not(self) -> Self::Output {
-        self.complement()
-    }
-}
-impl Add for &IntervalSet {
-    type Output = IntervalSet;
-    fn add(self, other: &IntervalSet) -> Self::Output {
-        self.union(other)
-    }
-}
-impl Sub for &IntervalSet {
-    type Output = IntervalSet;
-    fn sub(self, other: &IntervalSet) -> Self::Output {
-        self.difference(other)
-    }
-}
-impl BitOr for &IntervalSet {
-    type Output = IntervalSet;
-    fn bitor(self, other: &IntervalSet) -> Self::Output {
-        self.union(other)
-    }
-}
-impl BitAnd for &IntervalSet {
-    type Output = IntervalSet;
-    fn bitand(self, other: &IntervalSet) -> Self::Output {
-        self.intersection(other)
-    }
-}
 impl Not for IntervalSet {
-    type Output = IntervalSet;
-    fn not(self) -> Self::Output {
+    type Output = Self;
+    fn not(self) -> Self {
         self.complement()
     }
 }
 
+impl Add for IntervalSet {
+    type Output = Self;
+    fn add(self, other: Self) -> Self {
+        self.union(&other)
+    }
+}
+
+impl Sub for IntervalSet {
+    type Output = Self;
+    fn sub(self, other: Self) -> Self::Output {
+        self.difference(&other)
+    }
+}
+
+impl BitOr for IntervalSet {
+    type Output = Self;
+    fn bitor(self, other: Self) -> Self {
+        self.union(&other)
+    }
+}
+
+impl BitAnd for IntervalSet {
+    type Output = Self;
+    fn bitand(self, other: Self) -> Self {
+        self.intersection(&other)
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
diff --git a/src/resource_interval.rs b/src/resource_interval.rs
index ab36f15..38b82e9 100644
--- a/src/resource_interval.rs
+++ b/src/resource_interval.rs
@@ -5,12 +5,29 @@ use std::ops::{Add, Deref, DerefMut, Sub};
 /// represent where a resource is available, or where it's required
 /// Resources are independent, so overlaps between the
 /// interval sets are possible.
+#[derive(Debug, PartialEq)]
 pub struct ResourceInterval(HashMap<Resource, IntervalSet>);
 
 impl ResourceInterval {
-    fn new() -> Self {
+    pub fn new() -> Self {
         ResourceInterval(HashMap::new())
     }
+
+    pub fn insert(&mut self, other: ResourceInterval) {
+        for (res, is) in other.iter() {
+            self.0
+                .entry(res.clone())
+                .or_insert(IntervalSet::new())
+                .merge(is);
+        }
+    }
+
+    pub fn remove(&mut self, other: ResourceInterval) {
+        for (res, is) in other.iter() {
+            let avail = self.0.entry(res.clone()).or_insert(IntervalSet::new());
+            *avail = avail.difference(is);
+        }
+    }
 }
 
 impl Deref for ResourceInterval {
@@ -37,9 +54,9 @@ impl From<&HashMap<Resource, IntervalSet>> for ResourceInterval {
     }
 }
 
-impl Add for &ResourceInterval {
+impl Add for ResourceInterval {
     type Output = ResourceInterval;
-    fn add(self, other: &ResourceInterval) -> Self::Output {
+    fn add(self, other: ResourceInterval) -> Self::Output {
         let res: HashMap<Resource, IntervalSet> =
             other.0.iter().fold(self.0.clone(), |mut acc, (res, is)| {
                 acc.entry(res.clone())
@@ -51,16 +68,62 @@ impl Add for &ResourceInterval {
     }
 }
 
-impl Sub for &ResourceInterval {
+impl Sub for ResourceInterval {
     type Output = ResourceInterval;
-    fn sub(self, other: &ResourceInterval) -> Self::Output {
-        let res: HashMap<Resource, IntervalSet> =
-            other.0.iter().fold(self.0.clone(), |mut acc, (res, is)| {
-                acc.entry(res.clone())
-                    .or_insert(IntervalSet::new())
-                    .difference(is);
-                acc
-            });
+    fn sub(self, other: ResourceInterval) -> Self::Output {
+        let res: HashMap<Resource, IntervalSet> = self
+            .0
+            .iter()
+            .map(|(res, is)| {
+                (
+                    res.clone(),
+                    is.difference(other.get(res).unwrap_or(&IntervalSet::new())),
+                )
+            })
+            .collect();
         ResourceInterval(res)
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    macro_rules! intv {
+        ( $x:literal, $y:literal ) => {
+            Interval::new(
+                Utc.ymd(2022, 1, 1).and_hms($x, 0, 0),
+                Utc.ymd(2022, 1, 1).and_hms($y, 0, 0),
+            )
+        };
+    }
+
+    macro_rules! ri {
+        ( $r:literal, $(($x:literal, $y:literal)),* ) => {
+            ResourceInterval::from(HashMap::from([(
+                $r.to_owned(),
+                IntervalSet::from(vec![$(intv!($x, $y)),*]),
+            )]))
+        };
+    }
+
+    #[test]
+    fn test_conversion() {
+        let ri = ResourceInterval::from(HashMap::from([("alpha".to_owned(), IntervalSet::new())]));
+        assert_eq!(ri.len(), 1);
+    }
+
+    #[test]
+    fn test_addition() {
+        let a = ri!("alpha", (13, 15));
+
+        assert_eq!(a + ri!("alpha", (15, 18)), ri!("alpha", (13, 18)));
+    }
+
+    #[test]
+    fn test_subtraction() {
+        let a = ri!("alpha", (13, 18));
+
+        assert_eq!(a - ri!("alpha", (15, 16)), ri!("alpha", (13, 15), (16, 18)));
+    }
+}
diff --git a/src/task.rs b/src/task.rs
index 1283342..e51200f 100644
--- a/src/task.rs
+++ b/src/task.rs
@@ -157,7 +157,7 @@ impl Task {
             )]));
         self.provides.iter().all(|res| {
             if let Some(is) = available.get(res) {
-                !(&horizon_is - is).is_empty()
+                !(horizon_is.difference(is)).is_empty()
             } else {
                 false
             }