Function build_derived 

pub fn build_derived(
    reader: &Reader<'_>,
    args: &PlacedArgs,
) -> GeopResult<PlacedPart>

The part the program args places builds on its own — with its own state, placed nowhere: what a step deriving a part from another reads the bodies of. Shared with every build alike, a placed one’s too. Records the file and every file the part read.