diff --git a/src/module/mod.rs b/src/module/mod.rs
index 6d9681e6..305cdbc9 100644
--- a/src/module/mod.rs
+++ b/src/module/mod.rs
@@ -33,7 +33,7 @@ use crate::stdlib::{
     fmt, format,
     iter::empty,
     num::NonZeroUsize,
-    ops::{Deref, DerefMut},
+    ops::{Add, AddAssign, Deref, DerefMut},
     string::{String, ToString},
     vec::Vec,
 };
@@ -1546,6 +1546,38 @@ impl From<StaticVec<(String, Position)>> for ModuleRef {
     }
 }
 
+impl Add<Module> for Module {
+    type Output = Self;
+
+    fn add(self, rhs: Module) -> Self::Output {
+        let mut module = self.clone();
+        module.merge(&rhs);
+        module
+    }
+}
+
+impl Add<&Module> for Module {
+    type Output = Self;
+
+    fn add(self, rhs: &Module) -> Self::Output {
+        let mut module = self.clone();
+        module.merge(rhs);
+        module
+    }
+}
+
+impl AddAssign<Module> for Module {
+    fn add_assign(&mut self, rhs: Module) {
+        self.combine(rhs);
+    }
+}
+
+impl AddAssign<&Module> for Module {
+    fn add_assign(&mut self, rhs: &Module) {
+        self.merge(rhs);
+    }
+}
+
 impl ModuleRef {
     pub(crate) fn index(&self) -> Option<NonZeroUsize> {
         self.1
diff --git a/src/module/resolvers/collection.rs b/src/module/resolvers/collection.rs
index e513f51e..3c9eefa3 100644
--- a/src/module/resolvers/collection.rs
+++ b/src/module/resolvers/collection.rs
@@ -3,7 +3,7 @@ use crate::module::{Module, ModuleResolver};
 use crate::result::EvalAltResult;
 use crate::token::Position;
 
-use crate::stdlib::{boxed::Box, vec::Vec};
+use crate::stdlib::{boxed::Box, ops::AddAssign, vec::Vec};
 
 /// Module resolution service that holds a collection of module resolves,
 /// to be searched in sequential order.
@@ -45,9 +45,6 @@ impl ModuleResolversCollection {
     pub fn new() -> Self {
         Default::default()
     }
-}
-
-impl ModuleResolversCollection {
     /// Add a module keyed by its path.
     pub fn push(&mut self, resolver: impl ModuleResolver + 'static) {
         self.0.push(Box::new(resolver));
@@ -56,10 +53,29 @@ impl ModuleResolversCollection {
     pub fn iter(&self) -> impl Iterator<Item = &dyn ModuleResolver> {
         self.0.iter().map(|v| v.as_ref())
     }
+    /// Get a mutable iterator of all the modules.
+    pub fn into_iter(self) -> impl Iterator<Item = Box<dyn ModuleResolver>> {
+        self.0.into_iter()
+    }
     /// Remove all module resolvers.
     pub fn clear(&mut self) {
         self.0.clear();
     }
+    /// Is this `ModuleResolversCollection` empty?
+    pub fn is_empty(&self) -> bool {
+        self.0.is_empty()
+    }
+    /// Get the number of module resolvers in this `ModuleResolversCollection`.
+    pub fn len(&self) -> usize {
+        self.0.len()
+    }
+    /// Add another `ModuleResolversCollection` to the end of this collection.
+    /// The other `ModuleResolversCollection` is consumed.
+    pub fn append(&mut self, other: Self) {
+        if !other.is_empty() {
+            self.0.extend(other.0.into_iter());
+        }
+    }
 }
 
 impl ModuleResolver for ModuleResolversCollection {
@@ -83,3 +99,9 @@ impl ModuleResolver for ModuleResolversCollection {
         EvalAltResult::ErrorModuleNotFound(path.into(), pos).into()
     }
 }
+
+impl<M: ModuleResolver + 'static> AddAssign<M> for ModuleResolversCollection {
+    fn add_assign(&mut self, rhs: M) {
+        self.push(rhs);
+    }
+}
diff --git a/src/module/resolvers/stat.rs b/src/module/resolvers/stat.rs
index 563f5074..547bd5af 100644
--- a/src/module/resolvers/stat.rs
+++ b/src/module/resolvers/stat.rs
@@ -3,7 +3,7 @@ use crate::module::{Module, ModuleResolver};
 use crate::result::EvalAltResult;
 use crate::token::Position;
 
-use crate::stdlib::{boxed::Box, collections::HashMap, string::String};
+use crate::stdlib::{boxed::Box, collections::HashMap, ops::AddAssign, string::String};
 
 /// Module resolution service that serves modules added into it.
 ///
@@ -45,9 +45,6 @@ impl StaticModuleResolver {
     pub fn new() -> Self {
         Default::default()
     }
-}
-
-impl StaticModuleResolver {
     /// Add a module keyed by its path.
     pub fn insert<S: Into<String>>(&mut self, path: S, module: Module) {
         self.0.insert(path.into(), module);
@@ -68,6 +65,10 @@ impl StaticModuleResolver {
     pub fn iter_mut(&mut self) -> impl Iterator<Item = (&str, &mut Module)> {
         self.0.iter_mut().map(|(k, v)| (k.as_str(), v))
     }
+    /// Get a mutable iterator of all the modules.
+    pub fn into_iter(self) -> impl Iterator<Item = (String, Module)> {
+        self.0.into_iter()
+    }
     /// Get an iterator of all the module paths.
     pub fn paths(&self) -> impl Iterator<Item = &str> {
         self.0.keys().map(String::as_str)
@@ -84,6 +85,21 @@ impl StaticModuleResolver {
     pub fn clear(&mut self) {
         self.0.clear();
     }
+    /// Is this `StaticModuleResolver` empty?
+    pub fn is_empty(&self) -> bool {
+        self.0.is_empty()
+    }
+    /// Get the number of modules in this `StaticModuleResolver`.
+    pub fn len(&self) -> usize {
+        self.0.len()
+    }
+    /// Merge another `StaticModuleResolver` into this.
+    /// The other `StaticModuleResolver` is consumed.
+    pub fn merge(&mut self, other: Self) {
+        if !other.is_empty() {
+            self.0.extend(other.0.into_iter());
+        }
+    }
 }
 
 impl ModuleResolver for StaticModuleResolver {
@@ -94,3 +110,9 @@ impl ModuleResolver for StaticModuleResolver {
             .ok_or_else(|| EvalAltResult::ErrorModuleNotFound(path.into(), pos).into())
     }
 }
+
+impl AddAssign<Self> for StaticModuleResolver {
+    fn add_assign(&mut self, rhs: Self) {
+        self.merge(rhs);
+    }
+}
diff --git a/src/parser.rs b/src/parser.rs
index b2b30bb3..c83daba9 100644
--- a/src/parser.rs
+++ b/src/parser.rs
@@ -459,7 +459,7 @@ impl Add<&Self> for &AST {
     }
 }
 
-impl AddAssign<AST> for &mut AST {
+impl AddAssign<AST> for AST {
     fn add_assign(&mut self, rhs: AST) {
         self.combine(rhs);
     }