find-index

    function
    boot.janet on line 1304, column 1

    (find-index pred x &opt dflt)

    Find the index of `x` for which `pred` is true. If not found, 
    return `dflt` if provided, or `nil` otherwise.

    `x` can be a bytes, indexed, dictionary, or abstract type with 
    suitable `get` and `next` methods.