Macro pw_store_if_needed 

macro_rules! pw_store_if_needed {
    ($prog: ident, $pos: ident, $b: block) => { ... };
}

Internal helper macro for the code generator. It creates a ProgWriter instance from the given code block. It’s used where the ProgWriter needs it’s parent to define a place to put it’s result to, but it’s not required to be stored. $prog is the variable of the Prog.