Rewrite Tool allows you to create rewrite rule dynamically and see changes of result in the process of understandable creation of this rule.

Pharo 4.0

Metacello new 
    smalltalkhubUser: 'Pharo' project: 'MetaRepoForPharo40';
    configuration: 'RewriteTool';
    load

Pharo 5.0

Metacello new 
    smalltalkhubUser: 'MarkRizun' project: 'RewriteTool';
    configuration: 'RewriteTool';
    version: #development;
    load