Prepend shown unintentionally in editor

[amberrrruby]

Recreation:

1. Open up a mission that has predeclared primitives / functions, by clicking open through the sidebar.

2. After the editor loads in, you get access to the prepend section, by expanding from the first line // PREPEND

Unintended behaviour: For example, some predeclared features can be accessed and copied over for other assignments that does not contain such predeclared features.

PS This seems like it's already a known issue. Though I'm advised to raise it here for tracking purposes as a "vulnerability". (Hi Professor Martin)