Macro chain_rule

macro_rules! chain_rule {
    (first, Self::chain_rule($self:ident, $f0:expr, $f1:expr, $f2:expr, $f3:expr)) => { ... };
    (second, Self::chain_rule($self:ident, $f0:expr, $f1:expr, $f2:expr, $f3:expr)) => { ... };
    (third, Self::chain_rule($self:ident, $f0:expr, $f1:expr, $f2:expr, $f3:expr)) => { ... };
}