Trying to understand why my custom function seems to be ignoring most of it's arguments

Question

Defining a simple fibonacci function as:

fib=: verb : '{. (({: , +/ )^:y) 0 1'

I expected to be able to call it as

   fib 5 6 7   NB. expected 5 8 13

but instead I get

   fib 5 6 7
5 8

Trying to understand what is going on here