Enhance error handling in FactorDFG and VariableDFG constructors

src/entities/Factor.jl

@@ -128,6 +128,26 @@ function FactorDFG(
         timestamp = TimeDateZone(timestamp.utc_datetime)
     end
 
+    #TODO move to core constructor
+    if !isempty(multihypo) && length(multihypo) != length(variableorder)
+        throw(
+            ArgumentError(
+                "multihypo length ($(length(multihypo))) must match the number of variables ($(length(variableorder))). " *
+                "See fractional data-association uncertainty docs.",
+            ),
+        )
+    end
+    if length(variableorder) == 1 && observation isa AbstractRelativeObservation
+        throw(
+            ArgumentError(
+                "Relative observation $(typeof(observation)) requires at least two variables, but only one was provided: $variableorder. Use a subtype of AbstractPriorObservation for single-variable factors.",
+            ),
+        )
+    end
+    allunique(variableorder) || throw(
+        ArgumentError("Variable order must be unique, got duplicates in $variableorder"),
+    )
+
     # create factor data
     hyper = Recipehyper(; multihypo, nullhypo, inflation)
     state = Recipestate()

src/entities/Variable.jl

@@ -124,6 +124,7 @@ function VariableDFG(
     if solvable isa Int
         solvable = Ref(solvable)
     end
+    tags = tags isa Set ? tags : Set{Symbol}(tags)
     union!(tags, [:VARIABLE])
 
     P = getPointType(T)

src/services/compare.jl

@@ -215,7 +215,11 @@ function compareVariable(
     union!(skiplist, skip)
     # TP = TP && compareAll(A.states, B.states; skip = skiplist, show = show)
 
-    Ad = getState(A, :default) #FIXME why onlly comparing default?
+    #FIXME why only comparing hardcoded default?
+    if !hasState(A, :default) && !hasState(B, :default)
+        return false
+    end
+    Ad = getState(A, :default)
     Bd = getState(B, :default)
 
     # TP = TP && compareAll(A.attributes, B.attributes, skip=[:variableType;], show=show)