Macro __graph_internal 

macro_rules! __graph_internal {
    ($graph:expr => $left_node:ident: $output:ident => $right_node:ident: $input:ident,) => { ... };
    ($graph:expr => $left_node:ident => $right_node:ident: $input:ident,) => { ... };
    ($graph:expr => $left_node:ident => $right_node:ident,) => { ... };
}