Macro impl_interp 

macro_rules! impl_interp {
    (
        NAME = $name:ident
        FIELDS = ($($p:ident),*)
    ) => { ... };
    (
        NAME = $name:ident
        FIELD_TY = $f:ident
        FIELDS = ($($p:ident),*)
    ) => { ... };
    (
        NAME = $name:ident
        INDICES = [$($i:literal),*]
    ) => { ... };
}