pub fn partially_typed(&self) -> bool {
        use self::Type::Unknown;
        match self.ty {
            Unknown => true,
            _ => self.children().any(|e| e.ty.partial_type()),
        }
    }
Returns whether this `Expr` is partially typed.

A type is partial if it or any of its subtypes is `Unknown`.
weld/src/ast/ast.rs