This is mostly specific to Vyxal, but I've worded the language so that any hypothetical stack language could utilise this
Say you have a stack-based language which supports function objects on the stack.
And say that there's no way to take function objects on STDIN - they have to be placed in the header.
But say that there's a way to add function objects to STDIN via a built-in - it takes a function as its argument and injects it into where STDIN is coming from.
Is this a valid black-box function input format for challenges that require functions as input?