Fix mutation behavior of position

src/treetensornetworks/projttns/abstractprojttn.jl

@@ -133,10 +133,10 @@ function position(
 ) where {V}
   # shift position
   P = shift_position(P, pos)
-  # invalidate environments corresponding to internal edges
-  for e in internal_edges(P)
-    unset!(P.environments, e)
-  end
+  # remove internal edges (out of place)
+  ie = internal_edges(P)
+  newenvskeys = filter(!in(ie), keys(P.environments))
+  P = ProjTTN(pos, P.H, getindices(P.environments, newenvskeys))
   # make all environments surrounding new position
   for e in incident_edges(P)
     make_environment!(P, psi, e)

test/test_treetensornetworks/test_position.jl

@@ -0,0 +1,43 @@
+using ITensors
+using ITensorNetworks
+using ITensorNetworks: position
+using Test
+
+@testset "ProjTTN position copy-safe" begin
+  # make a nontrivial TTN state and TTN operator
+
+  auto_fermion_enabled = ITensors.using_auto_fermion()
+  use_qns = true
+  cutoff = 1e-12
+
+  tooth_lengths = fill(2, 3)
+  c = named_comb_tree(tooth_lengths)
+  if use_qns  # test whether autofermion breaks things when using non-fermionic QNs
+    ITensors.enable_auto_fermion()
+  else        # when using no QNs, autofermion breaks # ToDo reference Issue in ITensors
+    ITensors.disable_auto_fermion()
+  end
+  s = siteinds("S=1/2", c; conserve_qns=use_qns)
+
+  os = ITensorNetworks.heisenberg(c)
+
+  H = TTN(os, s)
+
+  d = Dict()
+  for (i, v) in enumerate(vertices(s))
+    d[v] = isodd(i) ? "Up" : "Dn"
+  end
+  states = v -> d[v]
+  psi = TTN(s, states)
+
+  # actual test, verifies that position is copy safe
+  vs = vertices(s)
+  PH0 = ProjTTN(H)
+  PH0 = position(PH0, psi, [vs[2]])
+  PH = copy(PH0)
+  PH = position(PH, psi, [vs[2], vs[5]])
+  @test keys(PH.environments) != keys(PH0.environments)
+  if !auto_fermion_enabled
+    ITensors.disable_auto_fermion()
+  end
+end