Fix ambiguity.

Author: samuelsonric

src/ambiguities.jl

@@ -30,6 +30,7 @@ permutation(weights::AbstractVector, alg::BT) = error()
 permutation(weights::AbstractVector, alg::SAT) = error()
 permutation(weights::AbstractVector, alg::MinimalChordal) = error()
 permutation(weights::AbstractVector, alg::CompositeRotations) = error()
+permutation(weights::AbstractVector, alg::GraphCompression) = error()
 permutation(weights::AbstractVector, alg::SafeRules) = error()
 permutation(weights::AbstractVector, alg::SafeSeparators) = error()
 permutation(weights::AbstractVector, alg::ConnectedComponents) = error()

src/elimination_algorithms.jl

@@ -2982,11 +2982,13 @@ function twins_impl!(
         @inbounds h = @view head[i]
         return DoublyLinkedList(h, prev, next)
     end
-    
-    s = popfirst!(free); m += one(V)
-    @inbounds prepend!(set(s), oneto(n)); size[s] = n
-    fill!(svar, s)
-    
+
+    if ispositive(n)    
+        s = popfirst!(free); m += one(V)
+        @inbounds prepend!(set(s), oneto(n)); size[s] = n
+        fill!(svar, s)
+    end    
+
     @inbounds for j in oneto(n)
         for i in neighbors(graph, j)
             if i != j

test/core.jl

@@ -477,6 +477,7 @@ end
             SAT{CryptoMiniSat_jll}(),
             MinimalChordal(),
             CompositeRotations([1, 2, 3]),
+            GraphCompression(),
             SafeRules(),
             SafeSeparators(),
             ConnectedComponents(),
@@ -558,6 +559,7 @@ end
             BT(),
             MinimalChordal(),
             CompositeRotations([]),
+            GraphCompression(),
             SafeRules(),
             SafeSeparators(),
             ConnectedComponents(),
@@ -620,6 +622,7 @@ end
             SafeFlowCutter(; time = 1),
             BT(),
             CompositeRotations([1]),
+            GraphCompression(),
             SafeRules(),
             SafeSeparators(),
             ConnectedComponents(),
@@ -907,6 +910,7 @@ end
                         BT(),
                         MinimalChordal(),
                         CompositeRotations([1, 3]),
+                        GraphCompression(),
                         SafeRules(),
                         SafeSeparators(),
                         ConnectedComponents(),