code: public boolean addClause(int lit, final Proposition proposition) {
      final LNGIntVector unit = new LNGIntVector(1);
      unit.push(lit);
      return this.addClause(unit, proposition);
    }
docs: Adds a unit clause to the solver.
  @param lit         the unit clause's literal
  @param proposition a proposition (if required for proof tracing)
  @return {@code true} if the clause was added successfully, {@code false} otherwise