Function child_file 

pub fn child_file(
    f: &ForStmt,
    comprehension: &Comprehension,
    elements: &[(String, PortType)],
    type_of: &dyn Fn(&str) -> Option<PortType>,
) -> Result<(PolydatFile, Vec<(String, PortType)>), String>

Build the child file for a traversal body: an implicit cycle input, one extern per element, one cascade extern per outer wire the body references and the parent exposes, then the body itself. Returns the file and the cascade list.