Y may be any array. R is the maximum number of levels of nesting of Y. A simple scalar (rank-0 number, character or namespace-reference) has a depth of 0.
A higher rank array, all of whose items are simple scalars, is termed a simple array and has a depth of 1. An array whose items are not all simple scalars is nested and has a depth 1 greater than that of its most deeply nested item.
Y is of uniform depth if it is simple or if all of its items have the same uniform depth.
If ⎕ML<2 and Y is not of uniform depth then R is negated (therefore, when ⎕ML<2, a negative value of R indicates non-uniform depth).
≡1 0 ≡'A' 0 ≡'ABC' 1 ≡1 'A' 1 ⎕ML←0 ≡A←(1 2)(3 (4 5)) ⍝ Non-uniform array ¯3 ≡¨A ⍝ A[1] is uniform, A[2] is non-uniform 1 ¯2 ≡¨¨A 0 0 0 1 ⎕ML←2 ≡A 3 ≡¨A 1 2 ≡¨¨A 0 0 0 1