diff --git a/masque/builder/devices.py b/masque/builder/devices.py
index 8d4875b..b3956bc 100644
--- a/masque/builder/devices.py
+++ b/masque/builder/devices.py
@@ -1,5 +1,5 @@
 from typing import Dict, Iterable, List, Tuple, Union, TypeVar, Any, Iterator, Optional, Sequence
-from typing import overload, KeysView, ValuesView
+from typing import overload, KeysView, ValuesView, MutableMapping
 import copy
 import warnings
 import traceback
@@ -46,7 +46,11 @@ class PortsRef(PortList):
         self.name = name
         self.ports = copy.deepcopy(ports)
 
-    def build(self, library: MutableLibrary) -> 'Builder':
+    def build(
+            self,
+            library: MutableLibrary,
+            tools: Union[None, Tool, MutableMapping[Optional[str], Tool]] = None,
+            ) -> 'Builder':
         """
         Begin building a new device around an instance of the current device
           (rather than modifying the current device).
@@ -56,7 +60,7 @@ class PortsRef(PortList):
         """
         pat = Pattern(ports=self.ports)
         pat.ref(self.name)
-        new = Builder(library=library, pattern=pat, tools=self.tools)   # TODO should Ref have tools?
+        new = Builder(library=library, pattern=pat, tools=tools)   # TODO should Ref have tools?
         return new
 
     # TODO do we want to store a Ref instead of just a name? then we can translate/rotate/mirror...
@@ -148,7 +152,7 @@ class Builder(PortList):
             library: MutableLibrary,
             pattern: Optional[Pattern] = None,
             *,
-            tools: Union[None, Tool, Dict[Optional[str], Tool]] = None,
+            tools: Union[None, Tool, MutableMapping[Optional[str], Tool]] = None,
             ) -> None:
         """
         If `ports` is `None`, two default ports ('A' and 'B') are created.
@@ -173,7 +177,7 @@ class Builder(PortList):
         elif isinstance(tools, Tool):
             self.tools = {None: tools}
         else:
-            self.tools = tools
+            self.tools = dict(tools)
 
         self._dead = False
 
@@ -546,12 +550,12 @@ class Builder(PortList):
             port_name = tuple(portspec)[0]
             return self.path(port_name, ccw, extensions[port_name], tool_port_names=tool_port_names)
         else:
-            bld = ports.as_interface(self.library, tools=self.tools)
+            bld = Pattern(ports=ports).as_interface(self.library, tools=self.tools)    # TODO: maybe Builder static as_interface-like should optionally take ports instead? Maybe constructor could do it?
             for port_name, length in extensions.items():
                 bld.path(port_name, ccw, length, tool_port_names=tool_port_names)
             name = self.library.get_name(base_name)
             self.library._set(name, bld.pattern)
-            return self.plug(PortsRef(name, pat.ports), {sp: 'in_' + sp for sp in ports.keys()})       # TODO safe to use 'in_'?
+            return self.plug(PortsRef(name, bld.pattern.ports), {sp: 'in_' + sp for sp in ports.keys()})       # TODO safe to use 'in_'?
 
     # TODO def path_join() and def bus_join()?
 
diff --git a/masque/builder/port_utils.py b/masque/builder/port_utils.py
index 1229b0e..3995de3 100644
--- a/masque/builder/port_utils.py
+++ b/masque/builder/port_utils.py
@@ -13,13 +13,13 @@ import numpy
 from ..pattern import Pattern
 from ..label import Label
 from ..utils import rotation_matrix_2d, layer_t
-from .devices import Device, Port
+from ..ports import Port
 
 
 logger = logging.getLogger(__name__)
 
 
-def dev2pat(device: Device, layer: layer_t) -> Pattern:
+def dev2pat(pattern: Pattern, layer: layer_t) -> Pattern:
     """
     Place a text label at each port location, specifying the port data in the format
         'name:ptype angle_deg'
@@ -27,24 +27,24 @@ def dev2pat(device: Device, layer: layer_t) -> Pattern:
     This can be used to debug port locations or to automatically generate ports
       when reading in a GDS file.
 
-    NOTE that `device` is modified by this function, and `device.pattern` is returned.
+    NOTE that `pattern` is modified by this function
 
     Args:
-        device: The device which is to have its ports labeled. MODIFIED in-place.
+        pattern: The pattern which is to have its ports labeled. MODIFIED in-place.
         layer: The layer on which the labels will be placed.
 
     Returns:
-        `device.pattern`
+        `pattern`
     """
-    for name, port in device.ports.items():
+    for name, port in pattern.ports.items():
         if port.rotation is None:
             angle_deg = numpy.inf
         else:
             angle_deg = numpy.rad2deg(port.rotation)
-        device.pattern.labels += [
+        pattern.labels += [
             Label(string=f'{name}:{port.ptype} {angle_deg:g}', layer=layer, offset=port.offset)
             ]
-    return device.pattern
+    return pattern
 
 
 def pat2dev(
@@ -53,10 +53,10 @@ def pat2dev(
         library: Optional[Mapping[str, Pattern]] = None,
         max_depth: int = 999_999,
         skip_subcells: bool = True,
-        ) -> Device:
+        ) -> Pattern:
     """
     Examine `pattern` for labels specifying port info, and use that info
-      to build a `Device` object.
+      to fill out its `ports` attribute.
 
     Labels are assumed to be placed at the port locations, and have the format
       'name:ptype angle_deg'
@@ -68,11 +68,12 @@ def pat2dev(
             Reduce this to 0 to avoid ever searching subcells.
         skip_subcells: If port labels are found at a given hierarcy level,
             do not continue searching at deeper levels. This allows subcells
-            to contain their own port info (and thus become their own Devices).
+            to contain their own port info without interfering with supercells'
+            port data.
             Default True.
 
     Returns:
-        The constructed Device object. Port labels are not removed from the pattern.
+        The updated `pattern`. Port labels are not removed.
     """
     ports = {}      # Note: could do a list here, if they're not unique
     annotated_cells = set()
@@ -109,5 +110,13 @@ def pat2dev(
 
         return pat
 
-    pattern.dfs(visit_before=find_ports_each, transform=True)       #TODO: don't check Library if there are ports in top level
-    return Device(pattern, ports)
+    # TODO TODO TODO
+    if skip_subcells and ports := find_ports_each(pattern, ...):
+        # TODO Could do this with just the `pattern` itself
+        pass
+    else
+        # TODO need `name` and `library` here
+        ports = library.dfs(name, visit_before=find_ports_each, transform=True)       #TODO: don't check Library if there are ports in top level
+    pattern.check_ports(other_ports=ports)
+    pattern.ports.update(ports)
+    return pattern
diff --git a/masque/builder/tools.py b/masque/builder/tools.py
index bd93f0c..1239100 100644
--- a/masque/builder/tools.py
+++ b/masque/builder/tools.py
@@ -4,7 +4,7 @@ Tools are objects which dynamically generate simple single-use devices (e.g. wir
 from typing import TYPE_CHECKING, Optional, Sequence
 
 if TYPE_CHECKING:
-    from .devices import Device
+    from ..pattern import Pattern
 
 
 class Tool:
@@ -17,6 +17,6 @@ class Tool:
             out_ptype: Optional[str] = None,
             port_names: Sequence[str] = ('A', 'B'),
             **kwargs,
-            ) -> 'Device':
+            ) -> 'Pattern':
         raise NotImplementedError(f'path() not implemented for {type(self)}')
 
diff --git a/masque/builder/utils.py b/masque/builder/utils.py
index 2a6507d..fff42e6 100644
--- a/masque/builder/utils.py
+++ b/masque/builder/utils.py
@@ -1,9 +1,10 @@
-from typing import Dict, Tuple, List, Optional, Union, Any, cast, Sequence, TYPE_CHECKING
+from typing import Dict, Tuple, List, Mapping, Sequence, SupportsFloat
+from typing import Optional, Union, Any, cast, TYPE_CHECKING
 from pprint import pformat
 
 import numpy
 from numpy import pi
-from numpy.typing import ArrayLike
+from numpy.typing import ArrayLike, NDArray
 
 from ..utils import rotation_matrix_2d
 from ..error import BuildError
@@ -135,6 +136,7 @@ def ell(
     #   D-----------|  `d_to_align[3]`
     #
     d_to_align = x_start.max() - x_start    # distance to travel to align all
+    offsets: NDArray[numpy.float64]
     if bound_type == 'min_past_furthest':
         #     A------------------V  `d_to_exit[0]`
         #               B-----V     `d_to_exit[1]`
@@ -154,6 +156,7 @@ def ell(
         travel = d_to_align - (ch_offsets.max() - ch_offsets)
         offsets = travel - travel.min().clip(max=0)
 
+    rot_bound: SupportsFloat
     if bound_type in ('emin', 'min_extension',
                       'emax', 'max_extension',
                       'min_past_furthest',):
diff --git a/masque/pattern.py b/masque/pattern.py
index 51805a0..3d72122 100644
--- a/masque/pattern.py
+++ b/masque/pattern.py
@@ -13,7 +13,7 @@ from numpy import inf
 from numpy.typing import NDArray, ArrayLike
 # .visualize imports matplotlib and matplotlib.collections
 
-from .refs import Ref
+from .ref import Ref
 from .shapes import Shape, Polygon
 from .label import Label
 from .utils import rotation_matrix_2d, normalize_mirror, AutoSlots, annotations_t
@@ -56,7 +56,7 @@ class Pattern(PortList, AnnotatableImpl, Mirrorable, metaclass=AutoSlots):
             labels: Sequence[Label] = (),
             refs: Sequence[Ref] = (),
             annotations: Optional[annotations_t] = None,
-            ports: Optional[Mapping[str, Port]] = None
+            ports: Optional[Mapping[str, 'Port']] = None
             ) -> None:
         """
         Basic init; arguments get assigned to member variables.
@@ -130,8 +130,17 @@ class Pattern(PortList, AnnotatableImpl, Mirrorable, metaclass=AutoSlots):
         self.refs += other_pattern.refs
         self.shapes += other_pattern.shapes
         self.labels += other_pattern.labels
-        self.annotations += other_pattern.annotations
-        self.ports += other_pattern.ports
+
+        annotation_conflicts = set(self.annotations.keys()) & set(other_pattern.annotations.keys())
+        if annotation_conflicts:
+            raise PatternError(f'Annotation keys overlap: {annotation_conflicts}')
+        self.annotations.update(other_pattern.annotations)
+
+        port_conflicts = set(self.ports.keys()) & set(other_pattern.ports.keys())
+        if port_conflicts:
+            raise PatternError(f'Port names overlap: {port_conflicts}')
+        self.ports.update(other_pattern.ports)
+
         return self
 
     def subset(
@@ -139,8 +148,8 @@ class Pattern(PortList, AnnotatableImpl, Mirrorable, metaclass=AutoSlots):
             shapes: Optional[Callable[[Shape], bool]] = None,
             labels: Optional[Callable[[Label], bool]] = None,
             refs: Optional[Callable[[Ref], bool]] = None,
-            annotations: Optional[Callable[[annotation_t], bool]] = None,
-            ports: Optional[Callable[[str], bool]] = None,
+            annotations: Optional[Callable[[str, List[Union[int, float, str]]], bool]] = None,
+            ports: Optional[Callable[[str, Port], bool]] = None,
             default_keep: bool = False
             ) -> 'Pattern':
         """
@@ -179,12 +188,12 @@ class Pattern(PortList, AnnotatableImpl, Mirrorable, metaclass=AutoSlots):
             pat.refs = copy.copy(self.refs)
 
         if annotations is not None:
-            pat.annotations = [s for s in self.annotations if annotations(s)]
+            pat.annotations = {k: v for k, v in self.annotations.items() if annotations(k, v)}
         elif default_keep:
             pat.annotations = copy.copy(self.annotations)
 
         if ports is not None:
-            pat.ports = {k: v for k, v in self.ports.items() if ports(k)}
+            pat.ports = {k: v for k, v in self.ports.items() if ports(k, v)}
         elif default_keep:
             pat.ports = copy.copy(self.ports)
 
diff --git a/masque/ports.py b/masque/ports.py
index fd9ffa3..0e0f377 100644
--- a/masque/ports.py
+++ b/masque/ports.py
@@ -1,5 +1,5 @@
-from typing import Dict, Iterable, List, Tuple, Union, TypeVar, Any, Iterator, Optional, Sequence
-from typing import overload, KeysView, ValuesView, ItemsView
+from typing import Dict, Iterable, List, Tuple, Iterator, Optional, Sequence, MutableMapping
+from typing import overload, KeysView, ValuesView, ItemsView, TYPE_CHECKING, Union, TypeVar, Any
 import copy
 import warnings
 import traceback
@@ -17,6 +17,9 @@ from .error import DeviceError
 from .library import MutableLibrary
 from .builder import Tool
 
+if TYPE_CHECKING:
+    from .builder import Builder
+
 
 logger = logging.getLogger(__name__)
 
@@ -117,10 +120,10 @@ class PortList(metaclass=ABCMeta):
         pass
 
     @overload
-    def __getitem__(self, key: Union[List[str], Tuple[str, ...], KeysView[str], ValuesView[str]]) -> PortList:
+    def __getitem__(self, key: Union[List[str], Tuple[str, ...], KeysView[str], ValuesView[str]]) -> Dict[str, Port]:
         pass
 
-    def __getitem__(self, key: Union[str, Iterable[str]]) -> Union[Port, PortList]:
+    def __getitem__(self, key: Union[str, Iterable[str]]) -> Union[Port, Dict[str, Port]]:
         """
         For convenience, ports can be read out using square brackets:
         - `pattern['A'] == Port((0, 0), 0)`
@@ -137,7 +140,7 @@ class PortList(metaclass=ABCMeta):
             return {k: self.ports[k] for k in key}
 
     # TODO add Mapping stuff to PortsList
-    def keys(self) -> KeysView[Port]:
+    def keys(self) -> KeysView[str]:
         return self.ports.keys()
 
     def values(self) -> ValuesView[Port]:
@@ -250,7 +253,7 @@ class PortList(metaclass=ABCMeta):
             self,
             library: MutableLibrary,
             *,
-            tools: Optional[Dict[str, Tool]] = None,
+            tools: Union[None, Tool, MutableMapping[Optional[str], Tool]] = None,
             in_prefix: str = 'in_',
             out_prefix: str = '',
             port_map: Optional[Union[Dict[str, str], Sequence[str]]] = None,
@@ -296,6 +299,8 @@ class PortList(metaclass=ABCMeta):
             `DeviceError` if applying the prefixes results in duplicate port
                 names.
         """
+        from .pattern import Pattern
+
         if port_map:
             if isinstance(port_map, dict):
                 missing_inkeys = set(port_map.keys()) - set(self.ports.keys())
@@ -319,7 +324,7 @@ class PortList(metaclass=ABCMeta):
         if duplicates:
             raise DeviceError(f'Duplicate keys after prefixing, try a different prefix: {duplicates}')
 
-        new = Builder(library=library, ports={**ports_in, **ports_out}, tools=tools)
+        new = Builder(library=library, pattern=Pattern(ports={**ports_in, **ports_out}), tools=tools)
         return new
 
     def find_transform(
diff --git a/masque/ref.py b/masque/ref.py
index b8f60b0..3553d2c 100644
--- a/masque/ref.py
+++ b/masque/ref.py
@@ -162,13 +162,13 @@ class Ref(
 
         return pattern
 
-    def rotate(self: S, rotation: float) -> S:
+    def rotate(self: R, rotation: float) -> R:
         self.rotation += rotation
         if self.repetition is not None:
             self.repetition.rotate(rotation)
         return self
 
-    def mirror(self: S, axis: int) -> S:
+    def mirror(self: R, axis: int) -> R:
         self.mirrored[axis] = not self.mirrored[axis]
         self.rotation *= -1
         if self.repetition is not None: