There is a certain risk, that the mustBeBooleanMagicIn method rewrite can crash the vm Issue 13805 <https://pharo.fogbugz.com/default.asp?13805> The receiver of the rewritten method is the "nonboolean" value receiving the ifTrue:ifFalse: message. But the rewritten method can include instvar accessor and self sends of the original method. We can include more rewrite rules in mustBeBooleanMagicIn like "rewrite instvar accessing" context receiver class instVarNamesAndOffsetsDo: [ :n :o | RBParseTreeRewriter new replace: n with: ('ThisContext receiver instVarAt: ', o asString); executeTree: methodNode. ]. "rewrite self sends" RBParseTreeRewriter new replace: 'self' with: 'ThisContext receiver'; executeTree: methodNode. But I don't know if this works for all possible situations. Or if there are better ways to do the rewriting.