Comments adressed

core/src/allocation.jl

@@ -300,7 +300,7 @@ function valid_sources(p::Parameters, allocation_network_id::Int)::Bool
             else
                 from_main_network = graph[id_source].allocation_network_id == 1
 
-                if !from_source_node & !from_main_network
+                if !from_source_node && !from_main_network
                     errors = true
                     @error "The source node of source edge $edge for subnetwork $allocation_network_id is neither a source node nor is it coming from the main network."
                 end

core/test/allocation_test.jl

@@ -181,20 +181,27 @@ end
     p = Ribasim.Parameters(db, cfg)
     (; allocation, graph) = p
     (; main_network_connections) = allocation
+
+    # Connections from main network to subnetworks
     @test isempty(main_network_connections[1])
     @test only(main_network_connections[2]) == (NodeID(2), NodeID(11))
     @test only(main_network_connections[3]) == (NodeID(6), NodeID(24))
     @test only(main_network_connections[4]) == (NodeID(10), NodeID(38))
 
+    # main-sub connections are part of main network allocation graph
     allocation_edges_main_network = graph[].edge_ids[1]
     @test Tuple{NodeID, NodeID}[(2, 11), (6, 24), (10, 38)] ⊆ allocation_edges_main_network
 
+    # Subnetworks interpreted as users require variables and constraints to
+    # support absolute value expressions in the objective function
     allocation_model_main_network = Ribasim.get_allocation_model(p, 1)
     problem = allocation_model_main_network.problem
     @test problem[:F_abs].axes[1] == NodeID[11, 24, 38]
     @test problem[:abs_positive].axes[1] == NodeID[11, 24, 38]
     @test problem[:abs_negative].axes[1] == NodeID[11, 24, 38]
 
+    # In each subnetwork, the connection from the main network to the subnetwork is
+    # interpreted as a source
     @test Ribasim.get_allocation_model(p, 2).problem[:source].axes[1] ==
           Tuple{NodeID, NodeID}[(2, 11)]
     @test Ribasim.get_allocation_model(p, 3).problem[:source].axes[1] ==