err::error_enum!

macro_rules! error_enum {
    (auto $enum_name:ident $elem:ident ( $elem_type:ty ) ) => { ... };
    (bare $enum_name:ident $elem:ident ( $($elem_type:ty),*) ) => { ... };
    ($(pub enum $enum_name:ident { $($elem_kind:ident $elem:ident ( $($elem_type:ty),* ) ),* })* ) => { ... };
}