minor details in schemes

<div id="comment:0"></div>

some about E241 and E502

CC:  @kliem @tscrim

Component: **refactoring**

Author: **Frédéric Chapoton**

Branch/Commit: **[`9410446`](https://github.com/sagemath/sagetrac-mirror/commit/94104462d5d78075dc37b6aeae3b14523bf91bbc)**

Reviewer: **Travis Scrimshaw**

_Issue created by migration from https://trac.sagemath.org/ticket/34644_

