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/test/api/sequence/test_unassign_sequence.py

@@ -0,0 +1,111 @@
+# IfcOpenShell - IFC toolkit and geometry engine
+# Copyright (C) 2021 Dion Moult <dion@thinkmoult.com>
+#
+# This file is part of IfcOpenShell.
+#
+# IfcOpenShell is free software: you can redistribute it and/or modify
+# it under the terms of the GNU Lesser General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# IfcOpenShell is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU Lesser General Public License for more details.
+#
+# You should have received a copy of the GNU Lesser General Public License
+# along with IfcOpenShell.  If not, see <http://www.gnu.org/licenses/>.
+
+import ifcopenshell.api.sequence
+import test.bootstrap
+
+
+# NOTE: sequence module features rely on entities introduced in IFC4
+# therefore no IFC2X3 tests
+class TestUnassignSequence(test.bootstrap.IFC4):
+    def _pair(self):
+        return (
+            ifcopenshell.api.sequence.add_task(self.file),
+            ifcopenshell.api.sequence.add_task(self.file),
+        )
+
+    def test_unassigning_a_sequence(self):
+        predecessor, successor = self._pair()
+        ifcopenshell.api.sequence.assign_sequence(
+            self.file, relating_process=predecessor, related_process=successor
+        )
+        ifcopenshell.api.sequence.unassign_sequence(
+            self.file, relating_process=predecessor, related_process=successor
+        )
+        assert len(self.file.by_type("IfcRelSequence")) == 0
+
+    def test_doing_nothing_if_the_tasks_are_not_sequenced(self):
+        predecessor, successor = self._pair()
+        ifcopenshell.api.sequence.unassign_sequence(
+            self.file, relating_process=predecessor, related_process=successor
+        )
+        assert len(self.file.by_type("IfcRelSequence")) == 0
+
+    def test_unassigning_every_sequence_between_the_pair_by_default(self):
+        predecessor, successor = self._pair()
+        for sequence_type in ("START_START", "FINISH_FINISH"):
+            ifcopenshell.api.sequence.assign_sequence(
+                self.file,
+                relating_process=predecessor,
+                related_process=successor,
+                sequence_type=sequence_type,
+            )
+        ifcopenshell.api.sequence.unassign_sequence(
+            self.file, relating_process=predecessor, related_process=successor
+        )
+        assert len(self.file.by_type("IfcRelSequence")) == 0
+
+    def test_unassigning_only_the_named_type(self):
+        predecessor, successor = self._pair()
+        for sequence_type in ("START_START", "FINISH_FINISH"):
+            ifcopenshell.api.sequence.assign_sequence(
+                self.file,
+                relating_process=predecessor,
+                related_process=successor,
+                sequence_type=sequence_type,
+            )
+        ifcopenshell.api.sequence.unassign_sequence(
+            self.file,
+            relating_process=predecessor,
+            related_process=successor,
+            sequence_type="START_START",
+        )
+        rels = self.file.by_type("IfcRelSequence")
+        assert len(rels) == 1
+        assert rels[0].SequenceType == "FINISH_FINISH"
+
+    def test_not_unassigning_a_type_that_is_not_there(self):
+        predecessor, successor = self._pair()
+        ifcopenshell.api.sequence.assign_sequence(
+            self.file,
+            relating_process=predecessor,
+            related_process=successor,
+            sequence_type="START_START",
+        )
+        ifcopenshell.api.sequence.unassign_sequence(
+            self.file,
+            relating_process=predecessor,
+            related_process=successor,
+            sequence_type="FINISH_FINISH",
+        )
+        assert len(self.file.by_type("IfcRelSequence")) == 1
+
+    def test_leaving_the_other_direction_alone(self):
+        task1, task2 = self._pair()
+        ifcopenshell.api.sequence.assign_sequence(
+            self.file, relating_process=task1, related_process=task2
+        )
+        ifcopenshell.api.sequence.assign_sequence(
+            self.file, relating_process=task2, related_process=task1
+        )
+        ifcopenshell.api.sequence.unassign_sequence(
+            self.file, relating_process=task1, related_process=task2
+        )
+        rels = self.file.by_type("IfcRelSequence")
+        assert len(rels) == 1
+        assert rels[0].RelatingProcess == task2