4.11.1 An extended form of conditional-update!

Some implementations of the Threads library may provide an extended form of conditional-update! for updating places other than locked variables. The implementation of this extended form requires the use of non-standard features in the Dylan macro system, and hence cannot be written as a portable macro. These non-standard extensions are subject to discussion amongst the Dylan language designers, and may eventually become features. Until such time as standardization occurs, implementations are not mandated to implement the extended form of conditional-update!, and portable code should not depend upon the feature.

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Common Dylan and Functional Extensions - 31 Mar 00