Function page_main_len 

pub fn page_main_len(program: &[Stmt<'_>], interner: &Interner) -> usize

How many leading statements of program are its page’s MAIN: everything before the first statement that acts on the page (drives_the_page) — the lowered prelude, the program’s definitions, and the top-level bindings a page’s State: may read. What follows is whoever drives the page, the page’s own mount among them. A reload runs exactly the main again.