diff --git a/fec/src/checkexp.c b/fec/src/checkexp.c
index a8a9af0..a1b3bb5 100644
--- a/fec/src/checkexp.c
+++ b/fec/src/checkexp.c
@@ -142,9 +142,20 @@ int lvalue_writable(FeCheckerState *s, FeNode *n)
         t=n->a ? n->a->sem_type : 0;
         if (t && t->kind==FE_TYPE_REF && n->b && n->b->text &&
             strcmp(n->b->text,"^")==0) return t->ref_mut;
+        /* Through an owner, what may be written is decided by what is owned,
+           not by whether the binding may be pointed somewhere else. `let p:
+           ^[]mut T` fixes p and leaves what it owns writable. */
+        if (t && t->kind==FE_TYPE_OWNED && n->b && n->b->text &&
+            strcmp(n->b->text,"^")==0)
+            return !t->elem || t->elem->kind!=FE_TYPE_SLICE || t->elem->ref_mut;
+        return lvalue_writable(s,n->a);
+    }
+    if (n->kind == FE_N_INDEX) {
+        /* An index into a slice asks the slice, not the binding. */
+        t=n->a ? n->a->sem_type : 0;
+        if (t && t->kind==FE_TYPE_SLICE) return t->ref_mut;
         return lvalue_writable(s,n->a);
     }
-    if (n->kind == FE_N_INDEX) return lvalue_writable(s,n->a);
     return 0;
 }
 
@@ -440,10 +451,16 @@ FeType *check_expr_core(FeCheckerState *s, FeNode *n)
             if (strcmp(n->a->b->text,"alloc_slice")==0) {
                 FeNode *count=arg ? arg->next : 0;
                 FeType *item;
-                if(!arg || arg->kind!=FE_N_IDENT || !count || count->next)
-                    err(c,n->loc,"mem.alloc_slice requires a type and length");
-                item=arg && arg->kind==FE_N_IDENT ?
-                    fe_type_intern(&c->types,arg->text) : unknown(c);
+                {
+                    /* The element type may be an instance -- `Slot(V)` -- and
+                       not just a name. */
+                    int named=0;
+                    item=arg ? type_from_expr(s,arg,&named) : unknown(c);
+                    if(!arg || !named || !count || count->next) {
+                        err(c,n->loc,"mem.alloc_slice requires a type and length");
+                        item=unknown(c);
+                    }
+                }
                 b=count ? check_expr(s,count) : unknown(c);
                 if(known(b) && !fe_type_is_integer(b))
                     err(c,count->loc,"slice length must be an integer");
@@ -578,9 +595,23 @@ FeType *check_expr_core(FeCheckerState *s, FeNode *n)
                 while(param && arg) {
                     a=check_expr(s,arg);
                     b=method_type(c,param->a,et);
-                    if(!compatible(b,a,arg) && a->kind!=FE_TYPE_UNKNOWN)
+                    /* A method argument gets the same call-only weakening a
+                       free function's does: an exclusive view may be handed
+                       over as a shared one for the length of the call, and an
+                       exclusive borrow is lent rather than given. */
+                    if(!compatible(b,a,arg) &&
+                       !(b && a && b->kind==FE_TYPE_SLICE &&
+                         a->kind==FE_TYPE_SLICE && !b->ref_mut && a->ref_mut &&
+                         fe_type_equal(b->elem,a->elem)) &&
+                       !(b && a && b->kind==FE_TYPE_REF && a->kind==FE_TYPE_REF &&
+                         !b->ref_mut && a->ref_mut &&
+                         fe_type_equal(b->elem,a->elem)) &&
+                       a->kind!=FE_TYPE_UNKNOWN)
                         err(c,arg->loc,"method argument type mismatch");
-                    mark_moved(s,arg,a);
+                    if(!call_reborrows(b,a) &&
+                       !(b && a && b->kind==FE_TYPE_SLICE &&
+                         a->kind==FE_TYPE_SLICE && !b->ref_mut && a->ref_mut))
+                        mark_moved(s,arg,a);
                     param=param->next;
                     arg=arg->next;
                 }
diff --git a/fec/src/checkgen.c b/fec/src/checkgen.c
index 45c1a31..619213f 100644
--- a/fec/src/checkgen.c
+++ b/fec/src/checkgen.c
@@ -226,9 +226,13 @@ FeType *build_struct_instance(FeCheck *c, FeUnit *home, FeNode *decl,
     for (f=decl->children;f;f=f->next) if (f->kind==FE_N_FIELD) ++fields;
     t->field_count=fields;
     if (fields) {
+        const char *save_unit=c->types.unit_name;
         t->fields=(FeFieldType *)fe_arena_alloc(&c->arena,
                                                 fields*sizeof(FeFieldType));
         if (!t->fields) { t->field_count=0; return t; }
+        /* Field types are written in the unit that declared the struct, not in
+           whichever unit asked for this instance. */
+        c->types.unit_name=home->name;
         push_instance_bindings(c,&save,t);
         bind_self(c,t);
         i=0;
@@ -240,6 +244,7 @@ FeType *build_struct_instance(FeCheck *c, FeUnit *home, FeNode *decl,
             ++i;
         }
         pop_bindings(c,&save);
+        c->types.unit_name=save_unit;
     }
     fe_type_layout_all(&c->types);
     /* A type that says how to let go of itself needs that method to exist for
diff --git a/fec/std/map.fe b/fec/std/map.fe
index 5e238c6..3427f19 100644
--- a/fec/std/map.fe
+++ b/fec/std/map.fe
@@ -1,4 +1,176 @@
-unit map;
-pub struct Map(K, V) {
+unit std.map;
+
+// A table from a run of bytes to a value.
+//
+// A compiler looks names up constantly and a linear scan over a list is the
+// wrong shape for that. Keys are copied into one buffer the map owns and each
+// slot records where in it the key sits -- the arena-and-handle shape R11 asks
+// for, which also means letting go of the map is two frees and not one per
+// entry.
+//
+// Open addressing with linear probing. The table is a power of two so the
+// index is a mask rather than a division, and it grows at three quarters full
+// because probing gets long well before the table gets full.
+
+pub struct Slot(V) {
+    at: usize,
     len: usize,
+    used: bool,
+    value: V,
+}
+
+pub struct Map(V) {
+    slots: ^[]mut Slot(V),
+    bytes: ^[]mut u8,
+    used_bytes: usize,
+    count: usize,
+
+    pub fn with_capacity(n: usize) -> !Self {
+        var room: usize = 8;
+        while room < n * 2 { room = room * 2; }
+        let table: ^[]mut Slot(V) = try mem.alloc_slice(Slot(V), room);
+        let text: ^[]mut u8 = try mem.alloc_slice(u8, 64);
+        var i: usize = 0;
+        while i < room {
+            table.^[i].used = false;
+            i = i + 1;
+        }
+        return Self{ slots: table, bytes: text, used_bytes: 0, count: 0 };
+    }
+
+    pub fn count_of(self: &Self) -> usize { return self.count; }
+
+    pub fn room(self: &Self) -> usize { return self.slots.^.n; }
+
+    /// Where `key` sits in the table: the slot holding it, or the first free
+    /// slot it could go in. Probing stops at a free slot, which is why a slot
+    /// is never cleared -- only ever filled.
+    fn find(self: &Self, key: []u8) -> usize {
+        let mask: usize = self.slots.^.n - 1;
+        var at: usize = hash(key) & mask;
+        while true {
+            if not self.slots.^[at].used { return at; }
+            if self.same(at, key) { return at; }
+            at = (at + 1) & mask;
+        }
+        return 0;
+    }
+
+    fn same(self: &Self, slot: usize, key: []u8) -> bool {
+        if self.slots.^[slot].len != key.n { return false; }
+        let from: usize = self.slots.^[slot].at;
+        var i: usize = 0;
+        while i < key.n {
+            if self.bytes.^[from + i] != key[i] { return false; }
+            i = i + 1;
+        }
+        return true;
+    }
+
+    pub fn has(self: &Self, key: []u8) -> bool {
+        return self.slots.^[self.find(key)].used;
+    }
+
+    pub fn get(self: &Self, key: []u8, missing: V) -> V {
+        let at: usize = self.find(key);
+        if self.slots.^[at].used { return self.slots.^[at].value; }
+        return missing;
+    }
+
+    pub fn put(self: &mut Self, key: []u8, value: V) -> !void {
+        if self.count * 4 >= self.slots.^.n * 3 { try self.regrow(); }
+        let at: usize = self.find(key);
+        if self.slots.^[at].used {
+            self.slots.^[at].value = value;
+            return;
+        }
+        try self.keep(key, at);
+        self.slots.^[at].value = value;
+        self.slots.^[at].used = true;
+        self.count = self.count + 1;
+        return;
+    }
+
+    /// Make sure `need` bytes fit, moving to a bigger buffer if they do not.
+    ///
+    /// Kept apart from `keep` because the borrow that swaps the buffer in is
+    /// a borrow of the whole of `self` -- borrowing is tracked at the root --
+    /// and it has to be over before any field is read again.
+    fn ensure_room(self: &mut Self, need: usize) -> !void {
+        let have: usize = self.bytes.^.n;
+        if need <= have { return; }
+        var room: usize = have;
+        while room < need { room = room * 2; }
+        let bigger: ^[]mut u8 = try mem.alloc_slice(u8, room);
+        var k: usize = 0;
+        let filled: usize = self.used_bytes;
+        while k < filled {
+            bigger.^[k] = self.bytes.^[k];
+            k = k + 1;
+        }
+        let old: ^[]mut u8 = mem.replace(&mut self.bytes, bigger);
+        mem.destroy(old);
+        return;
+    }
+
+    /// Copy `key` into the byte buffer and point the slot at it.
+    fn keep(self: &mut Self, key: []u8, slot: usize) -> !void {
+        let at: usize = self.used_bytes;
+        let need: usize = at + key.n;
+        try self.ensure_room(need);
+        var i: usize = 0;
+        while i < key.n {
+            self.bytes.^[at + i] = key[i];
+            i = i + 1;
+        }
+        self.slots.^[slot].at = at;
+        self.slots.^[slot].len = key.n;
+        self.used_bytes = at + key.n;
+        return;
+    }
+
+    /// Twice the slots, everything placed again. The keys do not move: they
+    /// live in the byte buffer and the slots only point at them.
+    fn regrow(self: &mut Self) -> !void {
+        let bigger: ^[]mut Slot(V) = try mem.alloc_slice(Slot(V),
+                                                        self.slots.^.n * 2);
+        let mask: usize = bigger.^.n - 1;
+        var i: usize = 0;
+        while i < bigger.^.n {
+            bigger.^[i].used = false;
+            i = i + 1;
+        }
+        i = 0;
+        while i < self.slots.^.n {
+            if self.slots.^[i].used {
+                let from: usize = self.slots.^[i].at;
+                let len: usize = self.slots.^[i].len;
+                var at: usize = hash(self.bytes.^[from..from + len]) & mask;
+                while bigger.^[at].used { at = (at + 1) & mask; }
+                bigger.^[at] = self.slots.^[i];
+            }
+            i = i + 1;
+        }
+        let old: ^[]mut Slot(V) = mem.replace(&mut self.slots, bigger);
+        mem.destroy(old);
+        return;
+    }
+
+    pub fn drop(self: &mut Self) -> void {
+        mem.destroy(self.slots);
+        mem.destroy(self.bytes);
+    }
+}
+
+/// FNV-1a. Small, fast, and good enough for identifiers; nothing here has to
+/// resist an adversary choosing the keys.
+pub fn hash(key: []u8) -> usize {
+    var h: u32 = 2166136261;
+    var i: usize = 0;
+    while i < key.n {
+        h = h ^ (key[i] as u32);
+        h = h * 16777619;
+        i = i + 1;
+    }
+    return h as usize;
 }
diff --git a/fec/tests/exec/maps.fe b/fec/tests/exec/maps.fe
new file mode 100644
index 0000000..c499f61
--- /dev/null
+++ b/fec/tests/exec/maps.fe
@@ -0,0 +1,48 @@
+// EXIT:0
+// OUTPUT:count 5
+// OUTPUT:fn 2 let 3 missing 0
+// OUTPUT:grown 64
+// OUTPUT:after 40
+// OUTPUT:balanced
+unit maps;
+import std.io;
+import std.map;
+import std.fmt;
+import std.sys;
+
+fn run() -> !void {
+    var seen: map.Map(i32) = try map.Map(i32).with_capacity(4);
+    try seen.put("unit", 1);
+    try seen.put("fn", 2);
+    try seen.put("let", 3);
+    try seen.put("struct", 4);
+    try seen.put("return", 5);
+    @print("count {}\n", seen.count_of());
+    @print("fn {} let {} missing {}\n", seen.get("fn", 0), seen.get("let", 0),
+           seen.get("nope", 0));
+    // Enough keys to make it grow more than once.
+    var buf: [8]u8 = undefined;
+    var i: usize = 0;
+    while i < 40 {
+        let n: usize = fmt.fmt_i32(buf[..], i as i32);
+        try seen.put(buf[0..n], (i as i32) + 100);
+        i = i + 1;
+    }
+    @print("grown {}\n", seen.room());
+    var found: usize = 0;
+    i = 0;
+    while i < 40 {
+        let n: usize = fmt.fmt_i32(buf[..], i as i32);
+        if seen.get(buf[0..n], 0) == (i as i32) + 100 { found = found + 1; }
+        i = i + 1;
+    }
+    @print("after {}\n", found);
+    return;
+}
+
+fn main() -> i32 {
+    run() catch |e| { @print("failed\n"); return 1; };
+    if sys.allocs() != sys.frees() { @print("leaked\n"); return 2; }
+    @print("balanced\n");
+    return 0;
+}