Wolfram Language & System 10.3 (2015)|Legacy Documentation
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assigns rhs to be the delayed value of lhs. rhs is maintained in an unevaluated form. When lhs appears, it is replaced by rhs, evaluated afresh each time.
- SetDelayed has attribute HoldAll, rather than HoldFirst.
- You can make assignments of the form , where test gives conditions for the applicability of each transformation rule. You can make several assignments with the same lhs but different forms of test.
- returns Null if the assignment specified can be performed, and returns $Failed otherwise.
Introduced in 1988