Identifies how the current function was called. 900⌶ applies only when called from within a variadic defined function (not a dfn).

Y may be any array.

The result R is Boolean. 1 means that the current function was called monadically; 0 means that it wasn't. If there is no function on the stack, the result is 0.

Example

     ∇ r←{left}foo right
[1]    r←900⌶⍬
     ∇
      foo 10
1
      0 foo 10
0