understanding this keyword in Alloy

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In the bellow code from Alloy book, what does this keyword refer to:

module library/list [t]
sig List {}
sig NonEmptyList extends List {next: List, element: t} 

...

fun List.first : t {this.element} 
fun List.rest : List {this.next} 
fun List.addFront (e: t): List {
    {p: List | p.next = this and p.element = e} 
}

I would appreciate if you give me an elaborate description of this usage in Alloy.

asked 36 secs ago

qartal

188

understanding this keyword in Alloy