Macro interpreter::modify

macro_rules! modify {
    ($heap:ident: -> $y:ident) => { ... };
    ($heap:ident: & $y:ident) => { ... };
    ($heap:ident: > $y:ident) => { ... };
    ($heap:ident: str $y:ident) => { ... };
    ($heap:ident: drop $y:ident) => { ... };
}