The ISO spec doesn't address what the index-of function should do when the left argument is not a scalar or a one-dimensional array.
GNU APL extends this, but I don't really understand in what way. How am I to interpret the output from this?
(2 2 ⍴ 104 105 106 107) ⍳ (3 4 ⍴ 100+⍳100)
┏→━━━━━━━━━━━━━━━━━━━━━━┓
↓┏⊖┓ ┏⊖┓ ┏⊖┓ ┏⊖┓ ┃
┃┃0┃ ┃0┃ ┃0┃ ┃0┃ ┃
┃┗━┛ ┗━┛ ┗━┛ ┗━┛ ┃
┃┏→━━┓ ┏→━━┓ ┏→━━┓ ┏→━━┓┃
┃┃0 0┃ ┃0 1┃ ┃1 0┃ ┃1 1┃┃
┃┗━━━┛ ┗━━━┛ ┗━━━┛ ┗━━━┛┃
┃┏⊖┓ ┏⊖┓ ┏⊖┓ ┏⊖┓ ┃
┃┃0┃ ┃0┃ ┃0┃ ┃0┃ ┃
┃┗━┛ ┗━┛ ┗━┛ ┗━┛ ┃
┗∊━━━━━━━━━━━━━━━━━━━━━━┛
Regards,
Elias