[R] What does class "call" mean? How do I make class "formula" into a "call"?

[Rita Carreira]

I have a list called "tabs" that I would like to have the same structure as my list "eqSystem." The two look like they have the same format but they are different because when I look at their attributes, class(eqSystem[[1]]) is "call" but class(tabs[[1]]) is "formula". I want to have class(tabs[[1]]) as a call too. So what does "call" mean? And how do I make an object of type "formula" be of type "call"? 
Thank you so much!!!--Rita
> class(tabs)
[1] "list"
> class(tabs[1])
[1] "list"
> class(tabs[[1]])
[1] "formula"> class(eqSystem)
[1] "list"
> class(eqSystem[1])
[1] "list"
> class(eqSystem[[1]])
[1] "call"


Rita
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