//! This will set `a` and `b` as the "root" values for
//! error-reporting, but actually operate on `c` and `d`. This is
//! sometimes useful when the types of `c` and `d` are not traceable
//! This will set `a` and `b` as the "root" values for
//! error-reporting, but actually operate on `c` and `d`. This is
//! sometimes useful when the types of `c` and `d` are not traceable