function test()

  lat_inds = siteinds("S=1/2", 4);
  state = ["Up" for i=1:4];
  ψ = productMPS(lat_inds, state);

  gates = ITensor[];
  push!(gates, op("H", lat_inds[1]));
  push!(gates, op("CNOT", lat_inds[1], lat_inds[2]));
  push!(gates, op("H", lat_inds[3]));
  push!(gates, op("CNOT", lat_inds[3], lat_inds[4]));
  push!(gates, op("CNOT", lat_inds[2], lat_inds[3]));
  push!(gates, op("H", lat_inds[2]));

  ϕ = apply(gates, ψ; cutoff=1e-8);

  gates_2 = ITensor[op("Proj0", lat_inds[2]), op("Proj0", lat_inds[3])];

  ϕ = apply(gates_2, ϕ; cutoff=1e-8);

  normalize!(ϕ)

  correlation_matrix(ϕ, "Proj0", "Proj0"; sites=[1, 4])
  correlation_matrix(ϕ, "Proj0", "Proj1"; sites=[1, 4])

end

ERROR: UndefVarError: `si` not defined in `ITensorMPS`
Suggestion: check for spelling errors or missing imports.
Stacktrace:
 [1] correlation_matrix(psi::MPS, _Op1::String, _Op2::String; sites::Vector{Int64}, site_range::Nothing, ishermitian::Nothing)
   @ ITensorMPS.jl/src/mps.jl:899
 [2] correlation_matrix
   @ ITensorMPS.jl/src/mps.jl:739 [inlined]
 [3] test()
   @ Main ./REPL[42]:33
 [4] top-level scope
   @ REPL[43]:1