Update
'open required projects' is a menuitem in the project-right-click-menu.
current problem:
I opened [i]bookstore[/i] manualy and added Libraries: right-click on [i]bookstore1[/i] -> properties -> Libraries -> add Library -> [i]TopLinkEssentials[/i] (and [i]hibernate jpa[/i] etc.)
Now I can see the jar-files in project-tab under [i]bookstore1[/i] > Libraries.
But (clean)build says still: '...bookstore/database/Book.java:13: package javax.persistence does not exist' (at least [i]bookstore[/i] is used )
The Editor find the packages: (after adding Libraries) the import is not red underlined - but build won't work.
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