adjust the teaching languages to use #true, #false, and '() instead of true false and empty
update history for true, false, empty change
fix some threading issues in set-language-level!

also, minor rackety
adjust docs for drracket/check-syntax so the example
doesn't include paths directly into the plt tree

please include on the release branch.
bring htdp/*sl languages up-to-date with #true, #false, '()
Remove types for interfaces from TR framework.

We don't yet support types for interface values.

Please merge to v6.1.1
Fix types for init arguments in framework

Please merge to v6.1.1
Fix a bug in typechecking kw function application

Mandatory kw arguments in function types could confuse
the typechecker when the function had the
`syntax-procedure-converted-arguments-property` property
set.
Add typed/net/git-checkout
Typo fixes in docs
fixed warning and tests
eliminated bug introduced with latest revision

please add to 6.1.1.
Fix types for foldl and foldr with 3 lists.

Thanks to Jack Firth for the report.
Work around a type alias bug in framework types

The type alias analysis is unable to detect a rare kind
of type dependency and as a result initializes type aliases
in an order that doesn't work. Adding the extra `Class`
boilerplate here accommodates the analysis.

Please merge to v6.1.1
Make: Dependency Manager. Documentation fix.
Add missing parenthese.
Fix type for stx->list in typed/syntax/stx

Please merge to v6.1.1
Fix TR treatment of parent row in class types

This bug manifested in having extraneous class
members in a type for a class whose superclass
expression was a mixin application.
Fix Guide typo

Please merge to v6.1.1
Fix TR docs on #:implements in Class types

Please merge to v6.1.1