diff --git a/src/cache.rs b/src/cache.rs
new file mode 100644
index 0000000..fcf7fc2
--- /dev/null
+++ b/src/cache.rs
@@ -0,0 +1,84 @@
+//! Caching utilities
+use std::mem::{self, MaybeUninit};
+use std::error;
+use std::borrow::Borrow;
+use std::sync::RwLock;
+
+/// A trait for objects that can cache an operation in themselves
+pub trait Cache
+{
+    type Cached;
+
+    /// Compute the `Cached` value.
+    fn cache(&self) -> Self::Cached;
+}
+
+/// A value that might be cached or not.
+#[derive(Debug, Clone, PartialEq, Eq, Hash, Ord, PartialOrd)]
+pub enum MaybeCached<T: Cache>
+{
+    Uncached(T),
+    Cached(T::Cached),
+}
+
+impl<T: Cache> MaybeCached<T>
+{
+    /// Cache the operation
+    #[inline] pub fn cache(&mut self)
+    {
+	match self {
+	    Self::Uncached(val) => *self = Self::Cached(val.cache()),
+	    _ => (),
+	}
+    }
+
+    /// Has this value been cached
+    pub fn is_cached(&self) -> bool
+    {
+	if let Self::Cached(_) = &self {
+	    true
+	} else {
+	    false
+	}
+    }
+
+    /// Consume into the cached operation
+    #[inline] pub fn into_cached(mut self) -> Self
+    {
+	self.cache();
+	self
+    }
+
+    /// Compute the operation
+    #[inline] pub fn compute(self) -> T::Cached
+    {
+	match self {
+	    Self::Cached(c) => c,
+	    Self::Uncached(val) => val.cache(),
+	}
+    }
+}
+
+impl<F, T> Cache for F
+where F: Fn() -> T
+{
+    type Cached = T;
+    #[inline] fn cache(&self) -> Self::Cached
+    {
+	self()
+    }
+}
+
+/*
+#[derive(Debug)]
+pub struct LazyCached<T: Cache + ?Sized>(RwLock<Option<T::Cached>>, T);
+
+impl<T: Cache> LazyCached<T>
+{
+    pub fn new(value: T) -> Self
+    {
+	Self(RwLock::new(None), value)
+    }
+}
+ */
+//TODO:idfk..
diff --git a/src/ext.rs b/src/ext.rs
index 50d05d6..bc4a558 100644
--- a/src/ext.rs
+++ b/src/ext.rs
@@ -341,12 +341,12 @@ impl<T> UnwrapErrInfallible<T> for Result<!, T>
     (priv $name:ident $(; $doc:literal)?) => {
 	$(#[doc=$doc])?
 	mod $name;
-	use $name::*;
+	use self::$name::*;
     };
     ($vis:vis $name:ident $(; $doc:literal)?) => {
 	$(#[doc=$doc])?
 	mod $name;
-	$vis use $name::*;
+	$vis use self::$name::*;
     };
     ($name:ident $(; $doc:literal)?) => ($crate::submod!(pub $name $(; $doc)?));
 }
diff --git a/src/main.rs b/src/main.rs
index 920815a..6f94157 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -38,6 +38,7 @@ fn install() -> eyre::Result<()>
     Ok(())
 }
 
+mod cache;
 mod ext;
 use ext::*;
 mod util;
diff --git a/src/server/user.rs b/src/server/user.rs
index 1a7e6e2..53c516a 100644
--- a/src/server/user.rs
+++ b/src/server/user.rs
@@ -31,6 +31,7 @@ pub struct User
     groups: Vec<GroupID>,
 }
 
+submod!(user; "impls for `User`.");
 
 //TODO: Iterator over all `Group`s a user is a part of (i.e. each group they're in's deduped & flattened inheritance graph)
 
diff --git a/src/server/user/user.rs b/src/server/user/user.rs
new file mode 100644
index 0000000..3e613e4
--- /dev/null
+++ b/src/server/user/user.rs
@@ -0,0 +1,113 @@
+//! impls for `User`
+use super::*;
+use std::borrow::Cow;
+use std::iter;
+
+/// A container of `Group` references in a `Userspace` from `GroupID`s
+#[derive(Debug, Clone)]
+pub struct Groups<'u>(usize, Cow<'u, [GroupID]>, &'u Userspace);
+/// An iterator over all groups a user is a part of.
+#[derive(Debug, Clone)]
+pub struct AllGroups<'u>(Vec<(Option<&'u Group>, GroupInheritanceIter<'u>)>);
+
+impl User
+{
+    /// All groups the user is explicitly a part of
+    pub fn groups_explicit<'a, 'u>(&'a self, space: &'u Userspace) -> Groups<'u>
+    where 'a: 'u
+    {
+	Groups(0, Cow::Borrowed(&self.groups[..]), space)
+    }
+
+    /// All groups the user is a part of, including inherited groups and implcit (todo) ones
+    pub fn all_groups<'a, 'u>(&'a self, space: &'u Userspace) -> AllGroups<'u>
+    where 'a: 'u
+    {
+	AllGroups(self.groups_explicit(space).map(|group| (Some(group), group.inherits_from(space))).rev().collect())
+    }
+}
+
+impl<'u>  Groups<'u>
+{
+    /// The group IDs remaining in this iterator.
+    pub fn ids(&self) -> &[GroupID]
+    {
+	&self.1[self.0..]
+    }
+    /// The userspace for this iterator.
+    pub fn space(&self) -> &Userspace
+    {
+	&self.2
+    }
+    /// Copy as a reference with a lower lifetime.
+    pub fn copied<'a>(&'a self) -> Groups<'a>
+    {
+	Groups(self.0, Cow::Borrowed(&self.1[..]), self.2)
+    }
+}
+impl<'u> Iterator for Groups<'u>
+{
+    type Item = &'u Group;
+    fn next(&mut self) -> Option<Self::Item>
+    {
+	(if self.0>=self.1.len() {
+	    None
+	} else {
+	    Some(self.2.groups.get(&self.1[self.0]).expect("Groups contained invalid group ID for its userspace"))
+	}, self.0+=1).0
+    }
+    #[inline] fn nth(&mut self, n: usize) -> Option<Self::Item>
+    {
+	if (..self.len()).contains(&n) {
+	    Some(self.2.groups.get(&self.1[n]).expect("Groups contained invalid group ID for its userspace"))
+	} else {
+	    None
+	}
+    }
+    fn size_hint(&self) -> (usize, Option<usize>) {
+	let v = self.1.len() - self.0;
+	(v, Some(v))
+    }
+}
+
+impl<'u> iter::DoubleEndedIterator for Groups<'u>
+{
+    fn next_back(&mut self) -> Option<Self::Item>
+    {
+	if self.0 == 0 {
+	    None
+	} else {
+	    self.0-=1;
+	    debug_assert!(self.0<self.1.len(), "bad DEI impl");
+	    Some(self.2.groups.get(&self.1[self.0]).expect("Groups contained invalid group ID for its userspace"))
+	}
+    }
+}
+impl<'u> iter::ExactSizeIterator for Groups<'u>{}
+impl<'u> iter::FusedIterator for Groups<'u>{}
+
+impl<'u> Iterator for AllGroups<'u>
+{
+    type Item = &'u Group;
+    fn next(&mut self) -> Option<Self::Item>
+    {
+	match self.0.last_mut()
+	{
+	    Some((mut v @ Some(_), _)) => v.take(),
+	    Some((None, iter)) => {
+		if let Some(igroup) = iter.next() {
+		    Some(igroup)
+		} else {
+		    self.0.pop();
+		    self.next()
+		}
+	    },
+	    _ => None
+	}
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+	(self.0.iter().map(|(g, i)| (g.is_some() as usize) + i.size_hint().0).sum(), None)
+    }
+}
+impl<'u> iter::FusedIterator for AllGroups<'u>{}
diff --git a/src/util.rs b/src/util.rs
index c72c54b..c8ed1b4 100644
--- a/src/util.rs
+++ b/src/util.rs
@@ -12,3 +12,9 @@ pub fn jitter<T>(min: T, max: T) -> T
     thread.sample(dist)
 }
 
+/// Compare pointer identity
+#[inline(always)] pub fn ptr_eq<T>(ptr: &T, other: &T) -> bool
+{
+    ptr as *const T as usize ==
+	other as *const T as usize
+}