BusinessProcess: Fix getPreviousPhaseDict() for BL with >1 children/parents BLs.

Called by TransformationSimulationRule and did not work with the
following use case:

  BL(trade_phase=TP1, successor=trade_state/TS1):
    * child_BL1(predecessor=trade_state/TS1,
                trade_phase=TP1/InventoryAccounting)
    * child_BL2(predecessor=trade_state/TS1,
                trade_phase=Transforming)

product/ERP5/Document/BusinessProcess.py

@@ -875,11 +875,15 @@ class BusinessProcess(Path, XMLObject):
         # i.e. edit next phases to replace current phase by previous ones
         for next in next_set:
           phase_set = result[next]
-          phase_set.remove(phase)
+          # Not remove() as it may have already been removed earlier
+          # if >1 elements of next_set have the same parent
+          phase_set.discard(phase)
           phase_set |= previous_set
         # and previous phases to replace current by next ones
         for previous in previous_set:
           phase_set = next_dict[previous]
-          phase_set.remove(phase)
+          # Not remove() as it may have already been removed earlier
+          # if >1 elements of previous_set have the same child
+          phase_set.discard(phase)
           phase_set |= next_set
     return result

product/ERP5/tests/testBPMCore.py

@@ -475,6 +475,67 @@ class TestBPMImplementation(TestBPMDummyDeliveryMovementMixin):
                       business_process.getRemainingTradePhaseList(
                        business_process.invoice))
 
+  def test_BusinessState_getPreviousTradePhaseDict(self):
+    """
+    Test for getPreviousTradePhaseDict() and use case for Business
+    Links with multiple children (in this test, deliver BL has 2
+    children: invoice and tax BL having deliver BL as predecessor).
+    """
+    category_tool = self.getCategoryTool()
+    business_process = self.createBusinessProcess()
+    business_link_order = self.createBusinessLink(business_process,
+                                 title='order', id='order',
+                                 trade_phase='default/order')
+    business_link_deliver = self.createBusinessLink(business_process,
+                                 title='deliver', id='deliver',
+                                 trade_phase='default/delivery')
+    business_link_invoice = self.createBusinessLink(business_process,
+                                 title='invoice', id='invoice',
+                                 trade_phase='default/invoicing')
+    business_link_tax = self.createBusinessLink(business_process,
+                                 title='tax', id='tax',
+                                 trade_phase='default/tax')
+    business_link_account = self.createBusinessLink(business_process,
+                                 title='accounting', id='account',
+                                 trade_phase='default/accounting')
+
+    trade_state = category_tool.trade_state
+    business_link_order.setSuccessorValue(trade_state.ordered)
+    business_link_deliver.setPredecessorValue(trade_state.ordered)
+    business_link_deliver.setSuccessorValue(trade_state.delivered)
+    business_link_invoice.setPredecessorValue(trade_state.delivered)
+    business_link_invoice.setSuccessorValue(trade_state.invoiced)
+    business_link_tax.setPredecessorValue(trade_state.delivered)
+    business_link_tax.setSuccessorValue(trade_state.invoiced)
+    business_link_account.setPredecessorValue(trade_state.invoiced)
+    business_link_account.setSuccessorValue(trade_state.accounted)
+
+    trade_phase = category_tool.trade_phase.default
+    def _u(trade_phase):
+      return trade_phase.getCategoryRelativeUrl()
+
+    self.assertEqual(
+      {_u(trade_phase.order): set(),
+       _u(trade_phase.delivery): {_u(trade_phase.order)},
+       _u(trade_phase.invoicing): {_u(trade_phase.delivery)},
+       _u(trade_phase.tax): {_u(trade_phase.delivery)},
+       _u(trade_phase.accounting): {_u(trade_phase.invoicing), _u(trade_phase.tax)}},
+      business_process.getPreviousTradePhaseDict())
+
+    self.assertEqual(
+      {_u(trade_phase.order): set(),
+       _u(trade_phase.invoicing): {_u(trade_phase.order)},
+       _u(trade_phase.accounting): {_u(trade_phase.invoicing), _u(trade_phase.order)}},
+      business_process.getPreviousTradePhaseDict(
+        trade_phase_list=[_u(trade_phase.order),
+                          _u(trade_phase.invoicing),
+                          _u(trade_phase.accounting)]))
+
+    self.assertEqual(
+      {_u(trade_phase.accounting): set()},
+      business_process.getPreviousTradePhaseDict(
+        trade_phase_list=[_u(trade_phase.accounting)]))
+
   def test_BusinessProcess_getExpectedTradeModelPathStartAndStopDate(self):
     """
     This test case is described for what start/stop date is expected on