code: public Variable variable(final String name) {
      Variable var = this.posLiterals.get(name);
      if (var == null) {
        var = new Variable(name, this);
        this.posLiterals.put(name, var);
      }
      return var;
    }
docs: Creates a new literal instance with a given name and positive phase.
  @param name the variable name
  @return a new literal with the given name and positive phase