Function replace_root 

pub fn replace_root<A, F: Fn(&A, &A) -> bool>(
    arity: usize,
    f: F,
    xs: &mut [A],
    x: A,
) -> A

Given a slice xs which is already a heap, replace the root. f(x,y) is whether x must be nearer to the root than y. xs being empty is an error.