Macro from 

macro_rules! from {
    () => { ... };
    ( $a1: expr ) => { ... };
    ( $a1: expr, $a2: expr ) => { ... };
    ( $a1: expr, $a2: expr, $a3: expr ) => { ... };
    ( $( $rest: expr ),* ) => { ... };
}

Macro to construct a struct from variadic arguments.