Macro eval 

macro_rules! eval {
    ($equation: expr) => { ... };
    ($equation: expr, true) => { ... };
    ($equation: expr, false) => { ... };
    ($equation: expr, vars: $vars: expr) => { ... };
    ($equation: expr, func: $func: expr) => { ... };
    ($equation: expr, true, vars: $vars: expr) => { ... };
    ($equation: expr, false, vars: $vars: expr) => { ... };
    ($equation: expr, true, func: $func: expr) => { ... };
    ($equation: expr, false, func: $func: expr) => { ... };
    ($equation: expr, vars: $vars: expr, func: $func: expr) => { ... };
    ($equation: expr, $deg: expr, $vars: expr, $func: expr) => { ... };
    ($equation: expr, $deg: expr, vars: $vars: expr, func: $func: expr) => { ... };
    ($equation: expr, deg: $deg: expr, vars: $vars: expr, func: $func: expr) => { ... };
}