Question: Where is elif not [n,x,y]::[nonnegint$3] documented?

In a recent question, the solution involved [...]::[list$3].

H:= proc(n, x, y)
    if n=0 then y+1
    elif n=1 and y=0 then x
    elif n=2 and y=0 then 0
    elif y=0 then 1
    elif not [n,x,y]::[nonnegint$3] then 
        'procname'(n-1, x, 'procname'(n, x, y-1))
        thisproc(n-1, x, thisproc(n, x, y-1))
end proc

Where is  the  form [...]::[list$3] documented?



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