def root_expression_for(fei)

      sfei = Ruote.sid(fei)

      exp = @expressions.find { |fe| sfei == Ruote.sid(fe.fei) }

      return nil unless exp
      return exp if exp.parent_id.nil?

      root_expression_for(exp.parent_id)
    end
def root_expression_for(fei)

      sfei = Ruote.sid(fei)

      exp = @expressions.find { |fe| sfei == Ruote.sid(fe.fei) }

      return nil unless exp
      return exp if exp.parent_id.nil?

      root_expression_for(exp.parent_id)
    end
Given an expression id, returns the root (top ancestor) for its
 expression.