warning: `ref` on an entire `let` pattern is discouraged, take a reference with `&` instead
  --> src/main.rs:35:15
   |
35 |       let rhs = stack![ bp.cross(&bv) - ap.cross(&av);
   |  _______________^
36 | |                       cp.cross(&cv) - ap.cross(&av) ];
   | |_____________________________________________________^
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#toplevel_ref_arg
   = note: `#[warn(clippy::toplevel_ref_arg)]` on by default
   = note: this warning originates in the macro `stack` (in Nightly builds, run with -Z macro-backtrace for more info)