{
  "name": "langford_n9__v7",
  "problem_type": "langford",
  "params": {
    "n": 9
  },
  "prompt": "Construct a Langford sequence L(2,9): a sequence of length 18 containing exactly 2 copies of each integer from 1 to 9, such that the two occurrences of each value v are exactly v+1 positions apart (so copies of 1 are 1 position apart, copies of 2 are 2 positions apart, etc.). Example: L(2,3) has solution [2, 3, 1, 2, 1, 3].\n\nReturn seq as a list of 18 integers in 1..9, or state \"UNSATISFIABLE\" if no such sequence exists.",
  "satisfiable": false,
  "solution": null,
  "difficulty": {
    "solve_time_ms": 2919.6,
    "search_space": 6402373705728000,
    "num_variables": 18,
    "num_constraints": 176,
    "num_edges": -1,
    "backend": "pycsp",
    "solve_tier": "hard",
    "solve_pct_global": 96.05,
    "solve_pct_type": 93.75
  },
  "partial_assignment": null
}