Assign / unassign sequence should be specific to sequence type

I previously incorrectly believed that sequence types are mutually exclusive. E.g. you can have max 1 relationship between two tasks. Now after looking at more schedules I realise it's logically allowed to have more than one sequence relationship. For example simultaneous SS + FF.

src/ifcopenshell-python/ifcopenshell/api/sequence/assign_sequence.py

@@ -106,8 +106,16 @@ def assign_sequence(
         # to be 2000-01-05.
         ifcopenshell.api.sequence.cascade_schedule(model, task=formwork)
     """
+    # Matched on the type as well as the pair. Two tasks may legitimately be
+    # sequenced more than once with different types — the classic case is a
+    # "ladder", where a start to start lets the follower begin once the leader
+    # has begun and a finish to finish stops it ending before the leader ends.
+    # Both constraints are real and neither implies the other, so matching on
+    # the pair alone silently hands back the wrong relationship: a caller adding
+    # a second sequence would instead be handed the first, and editing the type
+    # on it would destroy the constraint that was already there.
     for rel in related_process.IsSuccessorFrom or []:
-        if rel.RelatingProcess == relating_process:
+        if rel.RelatingProcess == relating_process and rel.SequenceType == sequence_type:
             return rel
     rel = file.create_entity(
         "IfcRelSequence",

src/ifcopenshell-python/ifcopenshell/api/sequence/duplicate_task.py

@@ -156,6 +156,7 @@ class Usecase:
                             self.file,
                             relating_process=relating_process,
                             related_process=related_process,
+                            sequence_type=inverse.SequenceType,
                         )
                         if inverse.TimeLag:
                             ifcopenshell.api.sequence.assign_lag_time(

src/ifcopenshell-python/ifcopenshell/api/sequence/unassign_sequence.py

@@ -19,17 +19,28 @@
 import ifcopenshell
 import ifcopenshell.api.sequence
 import ifcopenshell.util.element
+from typing import Optional
 
 
 def unassign_sequence(
     file: ifcopenshell.file,
     relating_process: ifcopenshell.entity_instance,
     related_process: ifcopenshell.entity_instance,
+    sequence_type: Optional[str] = None,
 ) -> None:
     """Removes a sequence relationship between tasks
 
+    Two tasks may be sequenced more than once with different types — see
+    :func:`ifcopenshell.api.sequence.assign_sequence` — so removing "the"
+    relationship between a pair is ambiguous. Left unspecified, every sequence
+    between the two is removed, which is what "make them unrelated" means and
+    what this did before the parameter existed. Name a type to remove only that
+    one and leave any others in place.
+
     :param relating_process: The previous / predecessor task.
     :param related_process: The next / successor task.
+    :param sequence_type: Optionally, remove only the sequence of this type.
+        Choose from FINISH_START, FINISH_FINISH, START_START, or START_FINISH.
     :return: None
 
     Example:
@@ -58,9 +69,12 @@ def unassign_sequence(
             relating_process=zone1, related_process=zone2)
     """
     for rel in related_process.IsSuccessorFrom or []:
-        if rel.RelatingProcess == relating_process:
-            history = rel.OwnerHistory
-            file.remove(rel)
-            if history:
-                ifcopenshell.util.element.remove_deep2(file, history)
+        if rel.RelatingProcess != relating_process:
+            continue
+        if sequence_type is not None and rel.SequenceType != sequence_type:
+            continue
+        history = rel.OwnerHistory
+        file.remove(rel)
+        if history:
+            ifcopenshell.util.element.remove_deep2(file, history)
     ifcopenshell.api.sequence.cascade_schedule(file, task=related_process)