code: @Override
    public Set<String> getVariables() {
      Set<String> variables = new HashSet<>();
      if(lhs.getVariable() != null) variables.add(lhs.getVariable());
      if(rhs.getVariable() != null) variables.add(rhs.getVariable());
  
      return variables;
    }
docs: Returns a set of variables referenced by the predicates
  @return set of variables