diff --git a/README.md b/README.md
index 23743e3..68c0686 100644
--- a/README.md
+++ b/README.md
@@ -233,5 +233,3 @@ my_pattern.ref(_make_my_subpattern(), offset=..., ...)
 * Tests tests tests
 * check renderpather
 * pather and renderpather examples
-* context manager for retool
-* allow a specific mismatch when connecting ports
diff --git a/examples/tutorial/pather.py b/examples/tutorial/pather.py
index a7aad68..101fbb5 100644
--- a/examples/tutorial/pather.py
+++ b/examples/tutorial/pather.py
@@ -265,6 +265,12 @@ def main() -> None:
     # when using pather.retool().
     pather.path_to('VCC', None, -50_000, out_ptype='m1wire')
 
+    # Now extend GND out to x=-50_000, using M2 for a portion of the path.
+    # We can use `pather.toolctx()` to temporarily retool, instead of calling `retool()` twice.
+    with pather.toolctx(M2_tool, keys=['GND']):
+        pather.path_to('GND', None, -40_000)
+    pather.path_to('GND', None, -50_000)
+
     # Save the pather's pattern into our library
     library['Pather_and_BasicTool'] = pather.pattern
 
diff --git a/masque/builder/builder.py b/masque/builder/builder.py
index b92c9ac..fed839a 100644
--- a/masque/builder/builder.py
+++ b/masque/builder/builder.py
@@ -2,7 +2,7 @@
 Simplified Pattern assembly (`Builder`)
 """
 from typing import Self
-from collections.abc import Sequence, Mapping
+from collections.abc import Iterable, Sequence, Mapping
 import copy
 import logging
 from functools import wraps
@@ -226,6 +226,7 @@ class Builder(PortList):
             inherit_name: bool = True,
             set_rotation: bool | None = None,
             append: bool = False,
+            ok_connections: Iterable[tuple[str, str]] = (),
             ) -> Self:
         """
         Wrapper around `Pattern.plug` which allows a string for `other`.
@@ -260,6 +261,11 @@ class Builder(PortList):
             append: If `True`, `other` is appended instead of being referenced.
                 Note that this does not flatten  `other`, so its refs will still
                 be refs (now inside `self`).
+            ok_connections: Set of "allowed" ptype combinations. Identical
+                ptypes are always allowed to connect, as is `'unk'` with
+                any other ptypte. Non-allowed ptype connections will emit a
+                warning. Order is ignored, i.e. `(a, b)` is equivalent to
+                `(b, a)`.
 
         Returns:
             self
@@ -293,6 +299,7 @@ class Builder(PortList):
             inherit_name=inherit_name,
             set_rotation=set_rotation,
             append=append,
+            ok_connections=ok_connections,
             )
         return self
 
diff --git a/masque/builder/pather.py b/masque/builder/pather.py
index 17d73b3..87f08a3 100644
--- a/masque/builder/pather.py
+++ b/masque/builder/pather.py
@@ -2,9 +2,10 @@
 Manual wire/waveguide routing (`Pather`)
 """
 from typing import Self
-from collections.abc import Sequence, MutableMapping, Mapping
+from collections.abc import Sequence, MutableMapping, Mapping, Iterator
 import copy
 import logging
+from contextlib import contextmanager
 from pprint import pformat
 
 import numpy
@@ -281,6 +282,37 @@ class Pather(Builder):
                 self.tools[key] = tool
         return self
 
+    @contextmanager
+    def toolctx(
+            self,
+            tool: Tool,
+            keys: str | Sequence[str | None] | None = None,
+            ) -> Iterator[Self]:
+        """
+          Context manager for temporarily `retool`-ing and reverting the `retool`
+        upon exiting the context.
+
+        Args:
+            tool: The new `Tool` to use for the given ports.
+            keys: Which ports the tool should apply to. `None` indicates the default tool,
+                used when there is no matching entry in `self.tools` for the port in question.
+
+        Returns:
+            self
+        """
+        if keys is None or isinstance(keys, str):
+            keys = [keys]
+        saved_tools = {kk: self.tools.get(kk, None) for kk in keys}      # If not in self.tools, save `None`
+        try:
+            yield self.retool(tool=tool, keys=keys)
+        finally:
+            for kk, tt in saved_tools.items():
+                if tt is None:
+                    # delete if present
+                    self.tools.pop(kk, None)
+                else:
+                    self.tools[kk] = tt
+
     def path(
             self,
             portspec: str,
diff --git a/masque/library.py b/masque/library.py
index 82277e1..90202dc 100644
--- a/masque/library.py
+++ b/masque/library.py
@@ -542,7 +542,7 @@ class ILibraryView(Mapping[str, 'Pattern'], metaclass=ABCMeta):
         Return:
             Topologically sorted list of pattern names.
         """
-        return list(TopologicalSorter(self.child_graph()).static_order())
+        return cast(list[str], list(TopologicalSorter(self.child_graph()).static_order()))
 
     def find_refs_local(
             self,
diff --git a/masque/pattern.py b/masque/pattern.py
index 1816762..0ae230d 100644
--- a/masque/pattern.py
+++ b/masque/pattern.py
@@ -1225,6 +1225,7 @@ class Pattern(PortList, AnnotatableImpl, Mirrorable):
             inherit_name: bool = True,
             set_rotation: bool | None = None,
             append: bool = False,
+            ok_connections: Iterable[tuple[str, str]] = (),
             ) -> Self:
         """
         Instantiate or append a pattern into the current pattern, connecting
@@ -1270,6 +1271,11 @@ class Pattern(PortList, AnnotatableImpl, Mirrorable):
             append: If `True`, `other` is appended instead of being referenced.
                 Note that this does not flatten  `other`, so its refs will still
                 be refs (now inside `self`).
+            ok_connections: Set of "allowed" ptype combinations. Identical
+                ptypes are always allowed to connect, as is `'unk'` with
+                any other ptypte. Non-allowed ptype connections will emit a
+                warning. Order is ignored, i.e. `(a, b)` is equivalent to
+                `(b, a)`.
 
         Returns:
             self
@@ -1300,6 +1306,7 @@ class Pattern(PortList, AnnotatableImpl, Mirrorable):
             map_in,
             mirrored=mirrored,
             set_rotation=set_rotation,
+            ok_connections=ok_connections,
             )
 
         # get rid of plugged ports
diff --git a/masque/ports.py b/masque/ports.py
index 6f029b1..1cc711a 100644
--- a/masque/ports.py
+++ b/masque/ports.py
@@ -419,6 +419,7 @@ class PortList(metaclass=ABCMeta):
             *,
             mirrored: bool = False,
             set_rotation: bool | None = None,
+            ok_connections: Iterable[tuple[str, str]] = (),
             ) -> tuple[NDArray[numpy.float64], float, NDArray[numpy.float64]]:
         """
         Given a device `other` and a mapping `map_in` specifying port connections,
@@ -435,6 +436,11 @@ class PortList(metaclass=ABCMeta):
                 port with `rotation=None`), `set_rotation` must be provided
                 to indicate how much `other` should be rotated. Otherwise,
                 `set_rotation` must remain `None`.
+            ok_connections: Set of "allowed" ptype combinations. Identical
+                ptypes are always allowed to connect, as is `'unk'` with
+                any other ptypte. Non-allowed ptype connections will emit a
+                warning. Order is ignored, i.e. `(a, b)` is equivalent to
+                `(b, a)`.
 
         Returns:
             - The (x, y) translation (performed last)
@@ -451,6 +457,7 @@ class PortList(metaclass=ABCMeta):
             map_in=map_in,
             mirrored=mirrored,
             set_rotation=set_rotation,
+            ok_connections=ok_connections,
             )
 
     @staticmethod
@@ -461,13 +468,14 @@ class PortList(metaclass=ABCMeta):
             *,
             mirrored: bool = False,
             set_rotation: bool | None = None,
+            ok_connections: Iterable[tuple[str, str]] = (),
             ) -> tuple[NDArray[numpy.float64], float, NDArray[numpy.float64]]:
         """
         Given two sets of ports (s_ports and o_ports) and a mapping `map_in`
           specifying port connections, find the transform which will correctly
           align the specified o_ports onto their respective s_ports.
 
-        Args:t
+        Args:
             s_ports: A list of stationary ports
             o_ports: A list of ports which are to be moved/mirrored.
             map_in: dict of `{'s_port': 'o_port'}` mappings, specifying
@@ -479,6 +487,11 @@ class PortList(metaclass=ABCMeta):
                 port with `rotation=None`), `set_rotation` must be provided
                 to indicate how much `o_ports` should be rotated. Otherwise,
                 `set_rotation` must remain `None`.
+            ok_connections: Set of "allowed" ptype combinations. Identical
+                ptypes are always allowed to connect, as is `'unk'` with
+                any other ptypte. Non-allowed ptype connections will emit a
+                warning. Order is ignored, i.e. `(a, b)` is equivalent to
+                `(b, a)`.
 
         Returns:
             - The (x, y) translation (performed last)
@@ -502,7 +515,8 @@ class PortList(metaclass=ABCMeta):
             o_offsets[:, 1] *= -1
             o_rotations *= -1
 
-        type_conflicts = numpy.array([st != ot and 'unk' not in (st, ot)
+        ok_pairs = {tuple(sorted(pair)) for pair in ok_connections if pair[0] != pair[1]}
+        type_conflicts = numpy.array([(st != ot) and ('unk' not in (st, ot)) and (tuple(sorted((st, ot))) not in ok_pairs)
                                       for st, ot in zip(s_types, o_types, strict=True)])
         if type_conflicts.any():
             msg = 'Ports have conflicting types:\n'
@@ -523,8 +537,8 @@ class PortList(metaclass=ABCMeta):
         if not numpy.allclose(rotations[:1], rotations):
             rot_deg = numpy.rad2deg(rotations)
             msg = 'Port orientations do not match:\n'
-            for nn, (k, v) in enumerate(map_in.items()):
-                msg += f'{k} | {rot_deg[nn]:g} | {v}\n'
+            for nn, (kk, vv) in enumerate(map_in.items()):
+                msg += f'{kk} | {rot_deg[nn]:g} | {vv}\n'
             raise PortError(msg)
 
         pivot = o_offsets[0].copy()
@@ -532,8 +546,8 @@ class PortList(metaclass=ABCMeta):
         translations = s_offsets - o_offsets
         if not numpy.allclose(translations[:1], translations):
             msg = 'Port translations do not match:\n'
-            for nn, (k, v) in enumerate(map_in.items()):
-                msg += f'{k} | {translations[nn]} | {v}\n'
+            for nn, (kk, vv) in enumerate(map_in.items()):
+                msg += f'{kk} | {translations[nn]} | {vv}\n'
             raise PortError(msg)
 
         return translations[0], rotations[0], o_offsets[0]
diff --git a/masque/shapes/arc.py b/masque/shapes/arc.py
index da5d3f8..67d932d 100644
--- a/masque/shapes/arc.py
+++ b/masque/shapes/arc.py
@@ -233,7 +233,7 @@ class Arc(Shape):
         r0, r1 = self.radii
 
         # Convert from polar angle to ellipse parameter (for [rx*cos(t), ry*sin(t)] representation)
-        a_ranges = self._angles_to_parameters()
+        a_ranges = cast(tuple[tuple[float, float], tuple[float, float]], self._angles_to_parameters())
 
         # Approximate perimeter via numerical integration
 
diff --git a/masque/utils/curves.py b/masque/utils/curves.py
new file mode 100644
index 0000000..b17b015
--- /dev/null
+++ b/masque/utils/curves.py
@@ -0,0 +1,95 @@
+import numpy
+from numpy.typing import ArrayLike, NDArray
+from numpy import pi
+
+
+def bezier(
+        nodes: ArrayLike,
+        tt: ArrayLike,
+        weights: ArrayLike | None = None,
+        ) -> NDArray[numpy.float64]:
+    """
+    Sample a Bezier curve with the provided control points at the parametrized positions `tt`.
+
+    Using the calculation method from arXiv:1803.06843, Chudy and Woźny.
+
+    Args:
+        nodes: `[[x0, y0], ...]` control points for the Bezier curve
+        tt: Parametrized positions at which to sample the curve (1D array with values in the interval [0, 1])
+        weights: Control point weights; if provided, length should be the same as number of control points.
+            Default 1 for all control points.
+
+    Returns:
+        `[[x0, y0], [x1, y1], ...]` corresponding to `[tt0, tt1, ...]`
+    """
+    nn = nodes.shape[0]
+    if weights is None:
+        weights = numpy.ones(nn)
+
+    t_half0 = tt <= 0.5
+    umul = tt / (1 - tt)
+    udiv = 1 / umul
+    umul[~t_half0] = 1
+    udiv[t_half0] = 1
+
+    hh = numpy.ones((tt.size, 1))
+    qq = nodes[None, 0] * hh
+    for kk in range(1, nn):
+        hh *= umul * (nn + 1 - kk) * weights[kk]
+        hh /= kk * udiv * weights[kk - 1] + hh
+        qq *= 1.0 - hh
+        qq += hh * nodes[None, kk]
+    return qq
+
+
+
+def euler_bend(switchover_angle: float) -> NDArray[numpy.float64]:
+    """
+    Generate a 90 degree Euler bend (AKA Clothoid bend or Cornu spiral).
+
+    Args:
+        switchover_angle: After this angle, the bend will transition into a circular arc
+            (and transition back to an Euler spiral on the far side). If this is set to
+            `>= pi / 4`, no circular arc will be added.
+
+    Returns:
+        `[[x0, y0], ...]` for the curve
+    """
+    # Switchover angle
+    # AKA: Clothoid bend, Cornu spiral
+    theta_max = numpy.sqrt(2 * switchover_angle)
+
+    def gen_curve(theta_max: float):
+        xx = []
+        yy = []
+        for theta in numpy.linspace(0, theta_max, 100):
+            qq = numpy.linspace(0, theta, 1000)
+            xx.append(numpy.trapz( numpy.cos(qq * qq / 2), qq))
+            yy.append(numpy.trapz(-numpy.sin(qq * qq / 2), qq))
+        xy_part = numpy.stack((xx, yy), axis=1)
+        return xy_part
+
+    xy_part = gen_curve(theta_max)
+    xy_parts = [xy_part]
+
+    if switchover_angle < pi / 4:
+        # Build a circular segment to join the two euler portions
+        rmin = 1.0 / theta_max
+        half_angle = pi / 4 - switchover_angle
+        qq = numpy.linspace(half_angle * 2, 0, 10) + switchover_angle
+        xc = rmin * numpy.cos(qq)
+        yc = rmin * numpy.sin(qq) + xy_part[-1, 1]
+        xc += xy_part[-1, 0] - xc[0]
+        yc += xy_part[-1, 1] - yc[0]
+        xy_parts.append(numpy.stack((xc, yc), axis=1))
+
+    endpoint_xy = xy_parts[-1][-1, :]
+    second_curve = xy_part[::-1, ::-1] + endpoint_xy - xy_part[-1, ::-1]
+
+    xy_parts.append(second_curve)
+    xy = numpy.concatenate(xy_parts)
+
+    # Remove any 2x-duplicate points
+    xy = xy[(numpy.roll(xy, 1, axis=0) != xy).any(axis=1)]
+
+    return xy